# Chapter 5 – Light

 Book Name : Physical Science and Environment Subject : Physical Science Class : 10 Madhyamik (WBBSE) Publisher : Oriental Book Company Private Limited Chapter Name : Light

Question – 1

What are the values of the angle of incidence and angle of reflection for normal incidence of light on a plane mirror?

When light is incident normally (perpendicular) on a plane mirror, the angle of incidence and the angle of reflection are both zero degrees.

Question – 2

No matter how far you stand from a mirror, your image always appears erect and diminished. What is the type of mirror?

Convex mirror.

Question – 3

Define the centre of curvature of a mirror.

The centre of curvature of a mirror is the centre of the sphere of which the spherical mirror is a part.

Question – 4

What is pole of a spherical mirror?

Pole is the midpoint of a spherical mirror.

Question – 5

Write the relation between radius of curvature and focal length of a concave mirror?

Radius of curvature = 2 × focal length

Question – 6

What kind mirror is need for shaving?

Concave mirror.

Explanation:

When an object placed close to a concave mirror, an erect and enlarged image is formed. So a concave mirror is used as a shaving mirror.

Question – 7

What is the refractive index of a medium?

The refractive index of a medium is defined as the ratio of the speed of light in the vacuum to that of the speed of light in the medium.

Question – 8

What happens when a ray of light strikes the surface of separation between the two media at a right angle?

The light ray passes without deviation.

Question – 9

What is the angle between the incident ray and the emergent ray of light, when an incident ray of light falls obliquely on the glass slab?

The incident ray and the emergent ray are parallel to to each other.

Question – 10

Define the refractive index of a medium.

The refractive index of a medium is defined as the ratio of the speed of light in the vacuum to that of the speed of light in the medium.

Question – 11

What is the focal length of a lens?

The distance between the optical centre and the focus of the lens is called the focal length of a lens.

Question – 12

What is a prism?

A transparent refractive medium bounded by five plane surface with a triangular cross-section is called a prism.

Question – 13

What is a lens?

A lens is a transparent refracting medium bounded by two curved surfaces which are usually spherical.

Question – 14

What is power of a lens?

The power of the lens is the reciprocal of the focal length of the lens.

Question – 15

What is the unit of power of a lens?

The unit of power of a lens Dioptre.

Question – 16

What is a photographic camera?

A photographic camera is a device for obtaining a permanent image of an object on a photographic plate or film.

Question – 17

Name two defects of the human eye.

Myopia (near-sightedness) and Hyperopia (farsightedness).

Question – 18

What are the distances between the near point and far point of a normal eye?

Near point: 25 cm

Far point: infinity.

Question – 19

State a difference between the images formed by the human eye and the photographic camera.

In a camera, the image formation is permanent, while the image formed by the human eye is temporary.

Question – 20

What is the dispersion of light?

The phenomenon of splitting white light into its seven constituting colours is called dispersion of light.

Question – 21

What is the range of visible light?

The range of visible light includes wavelengths that range from approximately 400 nanometers (nm) to 700 nm.

Question – 22

What is a scattering of light?

Scattering is the process of absorption of light energy and then re-emission of light energy by the dust particles and air molecules.

Question – 23

What is the nature of the image formed at the retina of the human eye?

Real and inverted

Question – 24

Name the part of the human eye which acts as a screen to obtain the image of an object.

Retina.

Question – 25

Name a phenomenon occurring in nature due to the dispersion of light.

Rainbow

Question – 26

Name the component of white light that deviates (i) the least and (i) the most while passing through a glass prism.

(i) Least deviating : Red

(ii) Most deviating : Violet

Question – 27

What colour of light is used for danger signal?

Red colour is used for danger signal.

Reason: Red has a long wavelength and is easily visible from a distance, even in low light conditions.

Question – 28

Draw a ray diagram to show the formation of images when the object is placed in front of a concave mirror

(i) between the pole and focus,

(ii) between the centre of curvature and focus.

• Position of object – Between focus and pole
• Position of the image – Behind the mirror
• Size of image – Magnified
• Nature of image – Virtual and upright

• Position of object – Between centre of curvature and focus
• Position of the image – Beyond centre of curvature
• Size of image – Magnified
• Nature of image – Real and inverted.

Question – 29

State three uses of a concave mirror.

Three uses of the concave mirror:-

(ii) Shaving mirrors

(iii) Denstist’s mirror

Question – 30

State two uses of a convex mirror.

Two uses of convex mirror :

(i) Rear view mirrors of cars and scooters

(ii) Street lamps

Question – 31

State laws of refraction of light.

Laws of refraction :

(i) The incident ray, the refracted ray and the normal at the point of incidence on the refracting surface lie in the same plane.

(ii) For a given pair of media and for a given colour of light, the sine of the angle of incidence bears a constant ratio to the sine of the angle of refraction. i.e sin i/ sin r =constant

Question – 32

With the help of a diagram, show that when light falls obliquely on a side of a rectangular glass slab, the emergent ray is parallel to the incident ray.

Question – 33

What is the difference between a convex lens and a concave lens?

Characteristic Convex Lens Concave Lens
Shape Thicker in the center, thin at the edges Thinner in the center, thicker at the edges
Focal Point Real or virtual, depending on the object’s position relative to the lens Virtual, always on the same side as the object
Principal Focus Positive focal length Negative focal length
Image Formation Forms real or virtual images, depending on the object’s position Forms only virtual, diminished images

Question – 34

The refractive index of a medium with respect to air is always greater than 1 Why?

Refractive index (μ) = c/v

c = Velocity of light in vacuum

v = Velocity of light in the medium

As the ‘c’ is always greater than ‘v’, therefore the refractive index of light of any material is always greater than 1.

Question – 35

Mention two properties of an image formed by a convex mirror.

Two properties of image formed by convex mirror :

(i) The image is always virtual and erect.

(ii) The image is highly diminished

Question – 36

What is the dispersion of light? What is its cause?

Dispersion of light: The phenomenon of splitting of white light by a prism into its constituent colours is known as dispersion.

Cause of dispersion: White colour is composed of seven colours and as each colour of light has its own wavelength, so different colours of light travel in a medium with different speeds. Therefore the cause of the dispersion of white light is due to the different speeds of light of different colours.

Question – 37

Why does the sky look blue?

The sky looks blue due to the scattering of sunlight by the molecules of the atmosphere. When sunlight passes through the atmosphere, then nitrogen and oxygen molecules present in the atmosphere absorb a part of sunlight and re-emit it. As the wavelength of the blue colour is shorter, it scatters more than any other colour light. Hence the sky looks blue.

Question – 38

Why are the stop signals on roads coloured red?

The wavelength of the red colour is the longest, therefore the light of the red colour is scattered least as compared to the light of other colours. Thus red light can be seen from the farthest distance in comparison to the light of other colours having the same intensity. Hence the stop signals on roads are coloured red.

Question – 39

Describe the action of a camera.

A photographic camera uses a convex lens of short focal length to capture a permanent image of an object on a light-sensitive film or photographic plate. The film is coated with silver halide, and when light falls on it, a chemical reaction occurs. Different amounts of light from various parts of the object affect different areas of the film, resulting in a negative image. The film is then chemically developed, and a positive print of the picture is obtained.

Question – 40

Describe how can the focal length of a concave mirror be determined experimentally.

A rough estimation of the focal length of a concave mirror can be done by holding a concave mirror facing a window in a room. The position and inclination of the mirror are so adjusted that a sharp image of the window is formed on the opposite wall. Parallel rays coming from the window falling On The Mirror would converge on the wall at the focus of the mirror. When a sharp image of the window is formed on the opposite wall. The distance between the mirror and the wall is determined by the scale. This gives the approximate focal length of the mirror.

Question – 41

Describe some uses of the spherical mirrors.

Here are some common uses of spherical mirrors:

1. Concave Mirrors:

• Dental Mirrors: Dentists often use small concave mirrors to view hard-to-reach areas inside the mouth.
• Makeup Mirrors: Some makeup mirrors have concave surfaces that magnify the reflection, allowing for precise makeup application.
• Reflecting Telescopes: In astronomy, concave mirrors are used as primary mirrors in reflecting telescopes to collect and focus light from distant celestial objects.

2. Convex Mirrors:

• Rearview Mirrors: Convex mirrors are commonly used as rearview mirrors in vehicles. They provide a wider field of view, making it easier to see vehicles behind and to the sides.
• Security Mirrors: Convex mirrors are used in stores, parking lots, and other areas to provide a wider field of view for security purposes, allowing people to see around corners and detect blind spots.

Question – 42

Deduce the relation, δ = i1 + i2 – A for refraction through a prism.

The angle of deviation (δ): The angle between the direction of the incident ray and the emergent ray is called the angle of deviation.

To prove δ = (i1 + i2) – A :

In fig, ∠ LMQ = ∠ MPQ + ∠ MQP

∴   Angle of deviation (δ) = δ1 + δ2 —- (i)

Since ∠ MPN = i1 and ∠ MQN = i2

∴   ∠ MPQ = δ1 = i1 – r1

and    ∠ MQP  = δ2 = i2 – r2

∴  From eq (i), δ = (i1 – r1) – (i2 – r2)

or,       δ = (i1 + i2) – (r1 + r2)—- (ii)

Also from the quadrilateral APNQ in fig

∠ APN = ∠ AQN = 90o

∴ ∠ PNQ + ∠ PAQ = 180o

or,   ∠ PNQ = 180o – A (∵ ∠ PAQ = A) —-(iii)

But in Δ PNQ,

∠ PNQ = 180o – (r1 + r2)

∴ From eq (ii) and (iii),

A =  r1 + r2 —-(iv)

Hence from eq (i) and (iv)

δ = (i1 + i2) – A (Proved).

Question – 43

Describe a few uses of spherical mirrors.

Here are a few uses of spherical mirrors described in points:

1. Convex mirrors are commonly used in cars as side mirrors to provide a wider field of view.
2. Concave mirrors are used in telescopes and satellite dishes to focus light and form images.
3. Security cameras often use convex mirrors as they provide a wider angle of view.
4. Dentists use concave mirrors to examine areas that are difficult to see in the mouth.
5. Concave mirrors are also used in lighthouses to focus light and improve visibility for sailors.
6. Headlights of cars often use concave mirrors to focus light and improve visibility for drivers.
7. Spherical mirrors are used in microscopes and other scientific instruments to magnify images and observe tiny objects.

Question – 44

Describe refraction through a prism and establish the relation δ = i1 + i2 – A.

The angle of deviation (δ): The angle between the direction of the incident ray and the emergent ray is called the angle of deviation.

To prove δ = (i1 + i2) – A :

In fig, ∠ LMQ = ∠ MPQ + ∠ MQP

∴   Angle of deviation (δ) = δ1 + δ2 —- (i)

Since ∠ MPN = i1 and ∠ MQN = i2

∴   ∠ MPQ = δ1 = i1 – r1

and    ∠ MQP  = δ2 = i2 – r2

∴  From eq (i), δ = (i1 – r1) – (i2 – r2)

or,       δ = (i1 + i2) – (r1 + r2)—- (ii)

Also from the quadrilateral APNQ in fig

∠ APN = ∠ AQN = 90o

∴ ∠ PNQ + ∠ PAQ = 180o

or,   ∠ PNQ = 180o – A (∵ ∠ PAQ = A) —-(iii)

But in Δ PNQ,

∠ PNQ = 180o – (r1 + r2)

∴ From eq (ii) and (iii),

A =  r1 + r2 —-(iv)

Hence from eq (i) and (iv)

δ = (i1 + i2) – A (Proved).

Question – 45

What is the scattering of light? Describe a natural phenomenon based on it.

Scattering: Scattering is the process of absorption of light energy and then re-emission of light energy by the dust particles and air molecules.

The sky looks blue due to the scattering of sunlight by the molecules of the atmosphere. When sunlight passes through the atmosphere, nitrogen and oxygen molecules present in the atmosphere absorb a part of sunlight and re-emit it. As the wavelength of the blue colour is shorter, it scatters more than any other colour light. Hence the sky looks blue.

Question – 46

Describe the formation of positions and nature of images formed by a concave mirror due to different positions of the object.

 Position of object Position of image Nature of image (i) Infinity At principal focus Real and inverted (ii) Beyond C Between F and C Real and inverted (iii) At C At C Real and inverted (iv) Between C and F Beyond C Real and inverted (v) At F At infinity Real and inverted (vi) Between P and F Behind the mirror Virtual and erect

Question – 47

Describe different uses of spherical mirrors.

Here are a few uses of spherical mirrors described in points:

1. Convex mirrors are commonly used in cars as side mirrors to provide a wider field of view.
2. Concave mirrors are used in telescopes and satellite dishes to focus light and form images.
3. Security cameras often use convex mirrors as they provide a wider angle of view.
4. Dentists use concave mirrors to examine areas that are difficult to see in the mouth.
5. Concave mirrors are also used in lighthouses to focus light and improve visibility for sailors.
6. Headlights of cars often use concave mirrors to focus light and improve visibility for drivers.
7. Spherical mirrors are used in microscopes and other scientific instruments to magnify images and observe tiny objects.

Question – 48

Describe refraction through a rectangular glass slab. Show the lateral shift produced by it.

Refraction through a rectangular glass slab occurs when a ray of light enters the slab at an angle other than perpendicular to the surface of the slab. The light wave changes speed as it enters the denser medium of the glass, causing it to bend towards the normal, which is a line perpendicular to the surface of the glass. When the light ray exits the glass slab, it bends away from the normal, since the light wave speeds up as it leaves the denser medium.

Question – 49

What are lenses? Describe the positions and nature of the images produced due to the different positions of the object.

 Position of object Position of image Nature of image (i) Infinity At f2 Real and inverted (ii) Beyond C Between f2 and 2f2 Real and inverted (iii) At C At 2f2 Real and inverted (iv) Between C and F Beyond 2f2 Real and inverted (v) At F At infinity Real and inverted (vi) Between P and F Same side of the object Virtual and erect

For concave lens

 Position of object Position of image Nature of image (i) Infinity At f1 Virtual and erect (ii) infinity and the optical centre Between f1 and the optical centre Virtual and erect

Question – 50

Distinguish and compare between the camera and human eyes.

 Human Eye Photographic Camera (a) Real, inverted image is formed on the retina. (b) The image cannot be stored as a photograph. (c) The focal length of the convex lens can be adjusted by ciliary muscles. (d) Eyes uses line cell to detect light. (a) Real, inverted image is formed on a film. (b) The image can be stored as a photograph. (c) The focal length of the lens cannot be adjusted. (d) Camera uses a diaphragm to detect light and capture images.

Question – 51

What is the dispersion of light? What is the cause of it?

Dispersion of light: The phenomenon of splitting of white light by a prism into its constituent colours is known as dispersion.

Cause of dispersion: White colour is composed of seven colours and as each colour of light has its own wavelength, so different colours of light travel in a medium with different speeds. Therefore the cause of the dispersion of white light is due to the different speeds of light of different colours.

Question – 52

What are electromagnetic waves? Describe a visible part of the spectrum.

Electromagnetic waves are a type of energy that travels through space. They are made up of both electric and magnetic fields that oscillate, or vibrate, in a synchronized manner. These waves don’t need any material medium to travel through; they can move through a vacuum, like outer space.

Visible Light: This is the part of the electromagnetic spectrum that our eyes can detect. It includes all the colours we see in a rainbow—red, orange, yellow, green, blue, indigo, and violet. Each colour corresponds to a different wavelength of light. Red has the longest wavelength, while violet has the shortest.

Question – 53

What is a scattering of light? Describe a natural phenomenon based on it.

Scattering: Scattering is the process of absorption of light energy and then re-emission of light energy by the dust particles and air molecules.

The sky looks blue due to the scattering of sunlight by the molecules of the atmosphere. When sunlight passes through the atmosphere, then nitrogen and oxygen molecules present in the atmosphere absorb a part of sunlight and re-emit it. As the wavelength of the blue colour is shorter, it scatters more than any other colour light. Hence the sky looks blue.

Question – 54

Describe why a flower appears green with white light and black with red light.

When white light falls on a flower, the pigments in the petals absorb certain colours of light and reflect others. In the case of a green flower, the pigments in the petals absorb all colours of light except for green. Green light is reflected, which is detected by our eyes, resulting in the perception of the flower as green.

On the other hand, when a flower is illuminated with red light, which contains only the red wavelengths of light, the pigments in the petals will absorb all the red light and not reflect any. Since the flower is not reflecting any light in this case, it appears black to our eyes.

Question – 55

Describe different parts of the human eye. Describe two major defects of the human eye and how they are remedied.

The human eye is a complex organ that consists of several parts, including:

1. Cornea: The transparent outer layer of the eye that helps to focus light.
2. Iris: The coloured part of the eye that regulates the amount of light that enters the eye.
3. Pupil: The black circular opening in the centre of the iris that allows light to enter the eye.
4. Lens: The clear, flexible structure located behind the iris helps to focus light on the retina.
5. Retina: The innermost layer of the eye that contains photoreceptor cells (rods and cones) that detect light and transmit visual signals to the brain.
6. Optic nerve: The nerve that carries visual signals from the retina to the brain.

Now, coming to the two major defects of the human eye and their remedies:

1. Myopia: Also known as nearsightedness, myopia is a condition in which a person can see objects clearly up close but has difficulty seeing objects that are far away. This occurs when the eyeball is too long or the cornea is too curved, causing light to be focused in front of the retina rather than on it.

Remedy: The most common remedy for myopia is wearing corrective eyeglasses or contact lenses that contain a concave lens, which helps to redirect light so that it focuses directly on the retina.

1. Hyperopia: Also known as farsightedness, hyperopia is a condition in which a person has difficulty seeing objects up close but can see distant objects clearly. This occurs when the eyeball is too short or the cornea is too flat, causing light to be focused behind the retina rather than on it.

Remedy: The most common remedy for hyperopia is wearing corrective eyeglasses or contact lenses that contain a convex lens, which helps to redirect light so that it focuses directly on the retina. In some cases, surgery may be performed to reshape the cornea and improve vision.

## Objective Questions:

Question – I

Fill in the blanks:

1. A convex mirror always produces a ____ image.
2. The refractive index of a medium depends upon ____ of the light.
3. A red flower appears ____ in green light.
4. The sky appears blue due to ____ of light.
5. In the human eye, the image is formed on ____.

1. Virtual
2. colour
3. black
4. scattering
5. retina

Question – II

State whether true or false:

1. A convex mirror is used as a driver’s mirror.
2. The absolute refractive index of a medium can be less than unity.
3. A band of seven colours of white light is called the spectrum.
4. In a photographic camera, a concave lens is used.
5. To correct long-sightedness a concave lens of suitable focal length is used.

1. True
2. False
3. True
4. False
5. False

Question – III

Match the following:

 Column I Column II (i) A sharing mirror (a) concave lens (ii) Always virtual, erect, diminished image (b) red light (iii) Blue of the sky (c) convex mirror (iv) Short sightedness (d) concave mirror (v) Danger signal (e) Scattering of light

(i) – (d)

(ii) – (c)

(iii) – (e)

(iv) – (a)

(v) – (b)

Question – IV

(i) Which of the following materials cannot be used to make a lens?

1. water
2. glass
3. clay
4. plastic

(ii) No matter how far you stand from a mirror, your image appears erect. The mirror is likely to be

1. plane only
2. concave only
3. convex only
4. either plane or convex

(iii) A ray of light suffers refraction through an equilateral prism. The deviation produced by the prism does not depend on the

1. angle of incidence
2. colour of light
3. size of the prism
4. material of the prism

(iv) A ray of light incident on a lens parallel to its principal axis, after refraction passes through or appears to come from

1. its second focus
2. its first focus
3. its optical centre
4. the centre of curvature of its second surface.

(v) The source of ultraviolet light is

1. electric bulb
2. carbon arc lamp
3. red hot iron bulb
4. sodium vapour lamp

(i) – (c)

(ii) – (d)

(iii) – (c)

(iv) – (a)

(v) – (b)

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