### All GRE Subject Test: Physics Resources

## 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 right of the lens

to the left of the lens

No image is created

to the left of the lens

to the right of the lens

**Correct answer:**

to the right of the lens

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?

**Possible Answers:**

, inverted

, inverted

No image is created.

, upright

, upright

**Correct answer:**

, inverted

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 #2 : 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 ?

**Possible Answers:**

**Correct answer:**

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 :

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