# 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?

Possible Answers:

to the left of the lens

to the right of the lens

to the right of the lens

No image is created

to the left of the lens

Correct answer:

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 #1 : 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?

Possible Answers:

, upright

, upright

, inverted

No image is created.

, inverted

Correct answer:

, 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 #1 : Optics

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 ?

Possible Answers:

Correct answer:

Explanation:

The Rayleigh Criterion gives the diffraction limit on resolution of a particular lens at a particular wavelength:

Where theta is the angular resolution in radians,  is the wavelength of light, and  is the diameter the lens in question. Solving for  and converting 3 arcseconds into radians, one can approximate the diameter to be about :