# Homework Help: Flash in cameras

1. Oct 18, 2011

### Celluhh

1. The problem statement, all variables and given/known data
how is flash created?

3. The attempt at a solution
flash makes images caught on a camera brighter and the background dark. why does this happen? what exactly is flash made up of? how do you explain the effects of flash?

2. Oct 18, 2011

### PeterO

Am I to assume you are referring to the flash on a camera?

3. Oct 18, 2011

### Celluhh

yes.

4. Oct 18, 2011

### PeterO

What does it look like comes out of a flash?

5. Oct 18, 2011

### Celluhh

white light rays?

6. Oct 18, 2011

### PeterO

Good.

Tell me about the intensity of light, and its variation over distance from the source.

7. Oct 18, 2011

### PeterO

Perhaps poor wording last time...
How does light intensity vary with distance?

8. Oct 18, 2011

### Celluhh

the greater the distance, the weaker the light intensity?

9. Oct 18, 2011

### PeterO

Yes it is weaker - and weaker in a definite way.

If you stood in front of a wall in sunlight - and could ignore all the extra light bouncing of the environment - you would be 150000 km from the sun, and the wall might be 150000.01 km fro the sun.

With a flash camera, you might be 3m from the flash, while the same wall would be 13m from the flash.

What would that mean?

EDIT: you better throw 3 more zeros in those "distance to the sun" figures!! ie 150000000 km

Last edited: Oct 18, 2011
10. Oct 19, 2011

### Celluhh

hi, can you explain it in simpler terms, i get the first part, but not the last part?

11. Oct 19, 2011

### NewtonianAlch

12. Oct 19, 2011

### PeterO

If the background is twice as far away as the subject, it will receive only 1/4 the light intensity.
You camera, which tries to automatically adjust for the light, will have the subject showing brightly, while the background is dark.

If you have a digital camera with flash, try it.
Have someone stand 2m from the camera in the following situations.
(a) against a wall.
(b) 2m from the wall.
(c) 4m from the wall.

if the last part was the "add three more zeros" it as because I understated the distance to the sun.

13. Oct 19, 2011

### Celluhh

ah, ok i get it now!thank you!

14. Oct 19, 2011

### DaveC426913

A secondary effect:

Because the subject is so brightly lit, the camera gets plenty of light for a properly exposed shot, and can stop down the exposure (higher f-stop, faster shutter). This mean the background, which might otherwise have been adequately lit, is now going to be very underexposed, i.e. black.

Much to the chagrin of many Moon hoaxers...