# Light travels in straight line

1. Apr 22, 2011

### logearav

1. The problem statement, all variables and given/known data

If an opaque object is placed between a source of light and a screen, a sharp shadow is obtained on the screen. This shows that light travels in straight lines. I don't understand this statement...How a sharp shadow on the screen proves that light travels in straight line?

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Apr 22, 2011

### Fewmet

Consider these two images:

The shadows edge in the former is quite distinct. If you were to draw lines from a point source of light, tangent to the edge of the sphere and onto the table top, they would form such a show. There would be a clear demarcation between where it is light and where it is dark.

If that line of light, however, were to bend a variable amount around the edge of the sphere, the border between light and dark would be more like what is shown in the latter image. It would grade from all dark to less dark to all light.

That soft edge happens in the second image because the light source is not a point or even a good approximation of a point. That gives an effect like the http://www.sems.und.edu/FrameA/LunarEclipses/lunar4.jpg" [Broken] in an eclipse. Note in the eclipse image that the straight lines of light give partial shadow in the penumbral region.

Last edited by a moderator: May 5, 2017
3. Apr 25, 2011

### logearav

Sir, i dont understand clearly. Can u give some other example, please?

4. Apr 25, 2011

### cupid.callin

let the light be coming from left andf making an image ap point P (there is a screen at P)
if now ou place a small object before P you will see a shadow (enlarged, depending on the distance b/w P and object)

now you can argue that light can turn a little and cross is object from side and then again meet at P ... making an image again ... which doesn't happen

so light cannot change its way on its own (coz its not forming an image)
therefore light must travel in straight line

5. Apr 26, 2011

### Fewmet

I am assuming you get that "sharp" shadow has very clear edge: you can say clearly "Here is the line where it stops being dark and starts being light". There is no gray between lit and unlit.

How would straight beams of light cause that? I think http://en.wikisource.org/wiki/File:PSM_V04_D690_Formation_of_a_shadow.jpg" [Broken] shows that well: those are straight lines of light that graze the edge of the sphere. The shadow is created where the light rays are prevented from falling. If light moved in a path that curved around the edges of the sphere, some light would fall in the area shown shadowed in that image.

Here is the problem with the answer: suppose the light did curve around the sphere. If all the light bent, it looks like there could still be a sharp (albeit smaller) shadow. So is this really evidence that light moves in a straight line?

Having thought through that by writing it, something else occurs to me. If the beams tangent to the sphere bent one degree and the "adjacent" beams went straight, there would be a gray ring around the shadow and the shadow would n longer be sharp. And if those beams bent, you could make the same argument about the next ones over...

Last edited by a moderator: May 5, 2017
6. Apr 27, 2011

### logearav

Now, I got it Mr. Fewmet. Thanks a lot.

7. Apr 27, 2011

### Fewmet

I am happy to help and glad that you asked. I've often seen the claim that sharp shadows mean light travels in a straight line, and did not question it as carefully as I should have. Your asking for clarification made me think more deeply and see the compelling truth of the statement.

8. Apr 27, 2011

### cupid.callin

but funny thing is that the statement "Light travels in straight line" is not completely true.

It will bend if it encounter an obstacle of size comparable to light's wavelength viz very small so we dont observe bending in daily life

9. Apr 27, 2011

### Fewmet

Yeah, I was danging around that...