NCERT Class X Science Solutions: Chapter 10 – Light, Reflection and Refraction Part 3

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Question 9:

One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.

Answer:

The convex lens will form complete image of an object, even if its one half is covered with black paper. It can be understood by the following two cases.

Case I

When the upper half of the lens is covered

In this case, a ray of light coming from the object will be refracted by the lower half of the lens. These rays meet at the other side of the lens to form the image of the given object, as shown in the following figure.

Result for When the upper half of the lens is covered.

When the Upper Half of the Lens is Covered

Case II

When the lower half of the lens is covered

In this case, a ray of light coming from the object is refracted by the upper half of the lens. These rays meet at the other side of the lens to form the image of the given object, as shown in the following figure.

Result for When the lower half of the lens is covered.

When the Lower Half of the Lens is Covered

Question 10:

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.

Answer:

Object distance,

Object height, ho

Focal length,

According to the lens formula,

The positive value of v shows that the image is formed at the other side of the lens.

The negative sign shows that the image is real and formed behind the lens.

The negative value of image height indicates that the image formed is inverted. The position, size, and nature of image are shown in the following ray diagram.

Result for converging lens

Converging Lens

Question 11:

A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray diagram.

Answer:

Focal length of concave lens

Image distance,

According to the lens formula,

The negative value of u indicates that the object is placed 30 cm in front of the lens. This is shown in the following ray diagram.

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