A glass prism bends a ray of blue light more than a ray of...

Name: Problem 78, Chapter 23: College Physics
Uploaded: 2021-09-28T20:11:48-08:00
Duration: 4 min 24 s
Channel: Linda Winkler
Description: A glass prism bends a ray of blue light more than a ray of red light since its index of refraction is slightly higher for blue than for red. Does a diverging glass lens have the same focal point for blue light and for red light? If not, for which color is the focal point closer to the lens?

A glass prism bends a ray of blue light more than a ray of red light since its index of refraction is slightly higher for blue than for red. Does a diverging glass lens have the same focal point for blue light and for red light? If not, for which color is the focal point closer to the lens?

Key Concepts

Chromatic Aberration

Chromatic aberration is an optical phenomenon where a lens refracts different wavelengths of light by different amounts, causing them to focus at different points. This is due to the variation of the refractive index with wavelength in the lens material, resulting in a separation of colors in the image.

Dispersion

Dispersion is the dependence of a material's refractive index on the wavelength of light. In optical materials, shorter wavelengths (such as blue light) generally have a higher refractive index than longer wavelengths (such as red light), leading to greater bending for the shorter wavelengths when passing through the material.

Diverging Lens Behavior

A diverging (or negative) lens causes incoming parallel rays to spread apart as if they were emanating from a virtual focal point on the same side of the lens as the light source. Due to dispersion, since blue light is refracted more strongly than red light, its virtual focal point is closer to the lens compared to that of red light.

Transcript

00:01 The index of refraction for many materials depends on the wavelength or the frequency of light that comes in.

00:08 This phenomena is known as dispersion.

00:13 For most materials like glass and plastic, the index of refraction for blue light is typically more than for something like red or longer wavelength light.

00:26 And what this means is that let's take a light gray of all incoming colors.

00:35 So this is white light coming in into a prism as shaped above.

00:43 White light.

00:45 What will happen is the light when it reaches the other side will bend away from the normal, but the blue light will bend more than the red light will.

00:58 And so the light coming out will be spread out into a rainbow spectrum.

01:06 Now, what does this have to do with the way that a thin lens would work? in particular, we're going to look at a diverging lens or a concave lens, if you want to think about it as concave shape.

01:29 But we're going to approximate that shape as if it were made of some very simple objects.

01:38 So we can place a prism up on the top.

01:45 We can do this with a converging lens as well, but here we'll just choose the same sort of simple shape that we did above.

01:53 There's a rectangle in between, and we'll put the prisms oriented the other way at the bottom.

02:03 This isn't really the way you would make a lens, but it is going to demonstrate what happens to the red and blue light...

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