# GRE Subject Test: Physics : Lenses

## Example Questions

### Example Question #1 : Lenses

The focal length of a thin convex lens is . A candle is placed  to the left of the lens. Approximately where is the image of the candle?

to the left of the lens

No image is created

to the right of the lens

to the left of the lens

to the right of the lens

to the right of the lens

Explanation:

Because the object is beyond 2 focal lengths of the lens, the image must be between 1 and 2 focal lengths on the opposite side. Therefore, the image must be between  on the right side of the lens.

Alternatively, one can apply the thin lens equation:

Where  is the object distance  and  is the focal length . Plug in these values and solve.

### Example Question #2 : Lenses

A candle  tall is placed  to the left of a thin convex lens with focal length . What is the height and orientation of the image created?

No image is created.

, upright

, inverted

, inverted

, upright

, inverted

Explanation:

First, find the image distance  from the thin lens equation:

Magnification of a lens is given by:

Where  and  are the image height and object height, respectively. The given object height is , which we can use to solve for the image height:

Because the sign is negative, the image is inverted.

### Example Question #3 : Lenses

Sirius is a binary star system, consisting of two white dwarfs with an angular separation of 3 arcseconds. What is the approximate minimum diameter lens needed to resolve the two stars in Sirius for an observation at ?