# 2 quick lens concept questions

## Homework Statement

1. A converging lens is used to focus parallel beams of red light and then later used to focus parallel beams of green light. Which focal length is longer? Explain

2. A photographer moves closer to his subject & then refocuses the camera. Is the camera lens now closer to or further from the film? Explain

## The Attempt at a Solution

I want to say red for the first one. I'm guessing that from snells law green light will converge quicker since it has a higher index of refraction than red light?

For the 2nd one I want to say further...Since the camera is closer the light coming in converges faster so the new focal point will be further away from the film?

## Answers and Replies

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mgb_phys
Homework Helper
"Blue Bent Best" - the green light focusses at a shorter distance than red.
So yes, the effective focal lenght for green light is shorter.

Correct again.
A useful rule to remember is that a lens at a distance 'f' focus light at infinity a lens at a distance 2f focuses light from a distance 2f.

"Blue Bent Best" - the green light focusses at a shorter distance than red.
So yes, the effective focal lenght for green light is shorter.

Correct again.
A useful rule to remember is that a lens at a distance 'f' focus light at infinity a lens at a distance 2f focuses light from a distance 2f.
thanks for the response!