NCERT Science Class 10 Chapter 10 Light Reflection and Refraction CBSE Board Sample Problems Long Answer

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Question

Draw ray diagrams showing the ¡mage formation by a concave mirror when an object is placed

(a) Between pole and focus of the mirror

(b) Between focus and centre of curvature of the mirror

(c) At centre of curvature of the mirror

(d) A little beyond centre of curvature of the mirror

(e) At infinity

Solution

a) Between pole and focus of the mirror

Ans.

Between pole and focus of the mirror

Between Pole and Focus of the Mirror

(b) Between focus and centre of curvature of the mirror

Ans.

Between focus and centre of curvature of the mirror

Between Focus and Centre of Curvature of the Mirror

(C) At centre of curvature of the mirror

Ans.

At centre of curvature of the mirror

At Centre of Curvature of the Mirror

(d) a little beyond centre of curvature of the mirror

Ans.

A little beyond centre of curvature of the mirror

A Little Beyond Centre of Curvature of the Mirror

(e) At infinity

Ans.

At infinity

At Infinity

Question

An object 5 cm in length is held 25 cm away from a converging lens of focal length 10 cm. Draw the ray diagram and find the position, size and the nature of the image formed.

Solution

Draw the ray diagram and find the position

Draw the Ray Diagram and Find the Position

The image is real and inverted at a distance of 16.7 cm from the lens on opposite side.

Magnification (m)

cm. Image s inverted and diminished.

Question

What are two possible positions of the object in case of a convex lens of focal length 20 cm in order to get an image twice the object height?

Solution

F= 20 cm

Case 1

When

Using lens formula we find u cm

Case 2

When m similarly

We get u=-30 cm

Question

(i) State laws of reflection and explain Snell’s law.

(ii)To construct a ray diagram we use two light rays which are so chosen that it is easy to know their directions after reflection from the mirror. List these two rays and state the path of these rays after reflection. Use these two rays to locate the image of an object placed between infinity and the pole of a convex mirror.

Solution

  • The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.

  • The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given colour and for the given pair of media. This law is also known as Snell’s law of refraction.

  • To construct a ray diagram we use two light rays: (a) A ray of light parallel to principal axis of a concave mirror, after reflection it passes through the principal focus of the concave mirror.

  • A ray of light which passes through the centre of curvature of a concave mirror, after reflection it retraces its path.

To construct a ray diagram we use two light rays

To Construct a Ray Diagram We Use Two Light Rays

Question

Draw the ray diagrams for all possible positions of the object in case of a concave mirror.

Solution

All possible positions of the object in case of a concave mi …

All Possible Positions of Object in Case of a Concave Mirror

Question

(a) Can a convergent lens in one medium become divergent in another medium?

(b) Two thin lenses of focal length +10 cm and -5 cm are kept in contact. What is the focal length and power of combination?

(C) A concave mirror of focal length of produces an image n times the size of an object.

What would be the object distance for which image is real?

Solution

(a) Yes, a convergent lens becomes divergent when the refractive index of the medium is greater than the refractive index of material of lens.

(b)

Power=

(In cm)

(c) Here, size of image / size of object= n

In case of concave mirror, if image is real then it must be inverted

From mirror formula

Question

Rohit claims to have obtained an image twice the size of object with a concave lens. Is he correct?

An object 2 cm high is placed at a distance of 16 cm from a concave mirror which produces a real image 3 cm high. Find the position of the image.

A real image, 4/5 size of the object is formed 18 cm from a lens. Calculate the focal length of the lens.

Solution

No, Rohit is incorrect because magnified image of an object cannot be formed by a concave lens ever.

Object height,

Image height, (real image is inverted)

Image is formed at a distance of 24 cm in front of the mirror.

3.

Therefore, u= -22.5 cm

Question

(a) A concave mirror is used as a head mirror by ENT specialists. Why and justify your answer with the help of ray diagram. (1+1)

(b) State mirror formula. How does 1’ change when object distance ‘u’ from the mirror is changed? (1)

(C) Rahul find the rough focal length of a convex lens. He is trying same method to find the focal length of concave lens; will he be successful. Justify your answer with help of ray diagram. (1+1)

Solution

A concave mirror is used as a head mirror by ENT specialists

A Concave Mirror is Used as a Head Mirror by ENT Specialists

(a) A concave mirror is used as doctor’s head mirror to focus light on body parts like eyes. Ears, nose, throat etc., to be examined. (1)

(b)

When u is changed, y changes, but f remains constant. This is because focal length of mirror depends only on radius of curvature of the mirror. (1)

(c)The rough focal length of a convex lens is obtained by forming sharp Image of a very distant object on a screen. The distance of the screen from the lens gives us the rough focal length of the lens. (1)

This method is not applicable to a concave lens, as image formed by a concave lens is virtual and it cannot be taken on a screen. (1)

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