Q1 of 84 Page 324

A virtual image three times the size of the object is obtained with a concave mirror of radius of curvature 36 cm. The distance of the object from the mirror is 

Given:

Magnification (m) = 3 (virtual image is erect).

Radius of the curvature, R =36 cm

Focal Length (f) = = -18cm (Negative for concave mirror).

Now by formula,

1. Magnification, 

Where,

v is the image distance,

u is the object distance,

Putting the values in the above formula, we get

⇒ 

2. Mirror formula

Where
f is the focal length of the mirror
v is the image distance from the mirror
u is the object distance from the mirror

Thus, putting the values in the equation, we get


Thus, on solving the above equation, we get


u=-12 cm

Hence the distance of the object from the mirror is 12 cm. 

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Varun’s mom is finding it difficult to cook in the kitchen as there was power cut and she was complaining about it to Varun. He immediately took a plane mirror from his shelf made it stand against a wall such that sun’s rays were focused into the kitchen directly on the cook top, due to the reflection of the mirror. There was some light now and his mother was able to finish her work.

(i) What are the characteristic of the image formed by a plane mirror?


(ii) What values did Varun display?

 40

Mr. Raghav, grade 6 teacher find that his students do not draw when he draws on the board. Every time he had to turn towards them to see whether they draw and label the diagrams. Arav, one of his student fixed two big convex mirrors at the two sides of the black board. With this Mr. Raghav was able to draw and also supervise his students through the mirror. All the students started doing their work property.

(i) Which characteristics of convex lens, let Mr. Raghav to do both his job at the same time?


(ii) What value did Arav and Mr. Raghav show?

 2

A convex mirror of focal length f forms an image of the size of the object. The distance of the object from the mirror is

 3

A perfecting reflecting mirror has an area of 1cm2. Light energy is allowed to fall on it for an hour at the rate of 10 Wcm-2. The force that acts on the mirror is