A student has to determine the focal length of a convex lens by obtaining the image of a distant object on a screen. Which of the following measures should he take to obtain better results?

a) Select a lens of small diameter (say $3 \mathrm{cm}$).

 b) Select a lens of larger diameter (say $5 \mathrm{cm}$).

 c) Select an object very far from the lens.

 d) Select an object far, but not very far, from the lens.

 e) Keep all the lights of the room on.

 f) Keep minimum lights of the room on.

To find the focal length of a concave mirror, Sita should choose which one of the following set-ups?

To find the focal length of a concave mirror, Ram focuses a distant object with this mirror. The chosen object should be—

A student obtained a sharp inverted image of a distant tree on a screen placed in front of the concave mirror. Then he removed the screen and tried to look into the mirror. He would now see _____.

Parallel light rays from a distant object incident on a device $\mathit{\text{'}}X\mathit{\text{'}}$ form a point size image on a screen. The device $\text{'}X\text{'}$ must be—

A student obtained a sharp image of the grills of a window on a screen using a concave mirror. His teacher remarked that for getting better results a well lit distant object (preferably the sun) should be focused on the screen. What should be done for this purpose?

A student obtains a blurred image of an object on a screen by using a convex lens. In order to obtain a sharp image of the same object on the screen, he will have to shift the lens:

A student performs an experiment on finding the focal length of a convex lens by keeping a lighted candle on one end of laboratory table, a screen on its other end and the lens between them as shown in figure.

The positions of the three are adjusted to get a sharp image of the candle flame on the screen.

If now the candle flame were to be replaced by a distant lamp on a far away electric pole, the student would be able to get a sharp image of this distant lamp on the screen by moving—

Mohan, Sohan and Rahul separately measured the focal length of the same convex lens using the same distant tree as object. All of them measured the distance between the lens and the inverted image of the tree on the screen. Mohan obtained a sharp image on the screen and labelled the measured distance as ${\mathrm{f}}_1$

Sohan obtained a slightly smaller and somewhat blurred image on the screen. He labelled the measured distance as ${\mathrm{f}}_2$

Rahul obtained a slightly magnified and blurred image on the screen. He labelled the measured distance as ${\mathrm{f}}_3$.

The three measured values of focal length are likely to be related as—