Light bending, also known as refraction, occurs when light waves travel from one transparent medium to another. The amount of bending depends on the refractive index of the two mediums. Refractive index measures how much light slows down as it passes through a material. The higher the refractive index, the more the light will bend as it travels from one medium to another.
What causes refraction?
Refraction occurs due to the change in speed that light experiences when moving between materials. Light waves travel fastest in a vacuum. When traveling through any material, the light slows down. How much it slows depends on the density of the material. Denser mediums like glass or water have higher refractive indexes. This means the speed of light is lower in these materials compared to air.
As light crosses the boundary between two mediums, one part of the wavefront slows down before the other part. This difference in speeds causes the wavefront to change direction, bending the path of the light ray. The amount of bending depends on the difference between the refractive indexes of the two materials. The greater the difference, the more the light will refract.
Do different colors of light bend differently?
Yes, the amount of refraction depends on the wavelength of light. Shorter wavelength violet and blue light will bend more than longer wavelength orange and red light. This is because refractive index also varies slightly with wavelength. All materials have a slightly higher refractive index for shorter wavelengths.
In glass, for example, the refractive index is about 1.52 for red light with a wavelength of 700 nm. For violet light at 400 nm, the refractive index is 1.54. This small difference is enough to cause measurable differences in refraction between colors.
Dispersion of white light
When white light passes through a prism, the different colors bend by different amounts, resulting in dispersion. Violet light bends the most, red bends the least. This separation of colors is how prisms produce rainbows from white light.
| Color | Wavelength (nm) | Refraction |
|---|---|---|
| Violet | 400 | High |
| Blue | 470 | High |
| Green | 510 | Medium |
| Yellow | 580 | Low |
| Orange | 620 | Low |
| Red | 700 | Lowest |
Measuring refractive index
The exact refractive index of a material depends on the wavelength of light passing through it. Refractive index is often measured using sodium light with a wavelength of 589 nm. At this wavelength, some common refractive index values are:
- Vacuum: 1.000
- Air: 1.0003
- Water: 1.333
- Glass: 1.517
- Diamond: 2.417
As the table shows, materials with a higher density and higher refractive index bend light more. Differences between mediums also affect refraction – a ray moving from air to glass bends more dramatically than from one type of glass to another.
Why dispersion occurs
Dispersion of light into colors happens because refractive index depends on wavelength. But why does refractive index vary with wavelength in the first place?
The speed of light in a material is influenced by interactions between light waves and electrons in the atoms. Shorter wavelengths contain more energy and interact more strongly, slowing down more. Ultraviolet light is slowed the most, infrared light the least.
Electrons vibrate more rapidly when hit by shorter wavelengths. This vibration dissipates some energy, reducing the light’s speed. The exact relationship between wavelength and refractive index depends on the atomic structure and electron configurations of the material.
Extent of dispersion
The variation in refractive index with wavelength is called dispersion. The extent of dispersion depends on the material:
| Material | Dispersion |
|---|---|
| Water | Low |
| Flint glass | High |
| Crown glass | Low |
| Diamond | High |
Materials with high dispersion separate colors more. Flint glass contains ions that interact strongly with different wavelengths, producing a broader range of refractive indexes.
Applications of dispersion
The dispersion of light is utilized in many optical devices and instruments:
- Prisms – Used to split white light into a rainbow spectrum for analysis.
- Glass lenses – Crown and flint glass reduce chromatic aberration.
- Fiber optics – Information carried on different wavelength channels.
- Spectroscopy – Dispersion allows identification of elements.
In some applications, dispersion needs to be minimized. Eyeglass lenses are made of materials with low dispersion to focus all colors on the retina.
Achromatic lenses
Normal lenses focus different wavelengths at different points due to dispersion. This chromatic aberration can be reduced by combining two types of glass:
- Crown glass has low dispersion
- Flint glass has high dispersion
By using both types of glass, the effects counteract each other. Colors are focused more closely together, reducing aberration. This technique is used in achromatic lenses for telescopes, microscopes, cameras, and glasses.
Achromatic doublet lens design
An achromatic doublet consists of a negative flint glass lens joined to a positive crown glass lens. The combined lens has reduced chromatic aberration:
| Lens | Material | Properties |
|---|---|---|
| Negative lens | Flint glass | – High dispersion – Diverges wavelengths |
| Positive lens | Crown glass | – Low dispersion – Converges wavelengths |
Even more wavelengths can be focused together using additional elements in more complex multi-lens objectives.
Why violet focuses differently
Out of all the colors, violet light focuses in a slightly different spot compared to other wavelengths. This occurs because the refractive index curve is not linear across the visible spectrum. It has a more pronounced “knee” in the blue/violet region.
Dispersion causes the colors to focus in order – red, orange, yellow, green, blue then violet. But violet deviates more than expected based on the trend. The steeper curve for violet means it focuses closer to the lens than blue or green light.
Refraction in the atmosphere
Refraction affects visible light passing through the atmosphere due to variations in air density. As light moves from low to high density, the path bends towards the denser medium. This occurs because gases have a slightly higher refractive index at higher pressures.
Effects of atmospheric refraction include:
- Sunlight bending towards Earth, allowing the sun to remain visible even when below the horizon.
- Stars and planets appearing slightly higher in the sky than their true position.
- Variations in density causing twinkling of stars.
Atmospheric refraction also separates light into colors, but scattering has a greater effect. More blue light is scattered, causing the red sun at sunset.
Refraction in rainbow formation
Refraction is responsible for rainbows produced by sunlight interacting with rain droplets. Here’s how rainbows form:
- Sunlight enters spherical raindrop, slowing down and bending upon entry.
- Light reflects off inside surface, bouncing back towards entry point.
- Light slows again and bends when exiting droplet.
- Dispersion causes different coloring based on refraction.
- Rainbow pattern forms with red on outside and violet on inside.
A double rainbow can sometimes be seen. This is caused by light reflecting twice inside raindrops before exiting.
Rainbow colors
| Color | Angle |
|---|---|
| Red | 42° |
| Orange | 40° |
| Yellow | 39° |
| Green | 38° |
| Blue | 36° |
| Violet | 34° |
The angular distance between red and violet light exiting the raindrops causes the rainbow spectrum pattern.
Refraction in mirages
Mirages form when temperature differences cause light rays to bend over long distances. There are two main types of mirages:
Inferior mirage
Hot air near the ground refracts light from the sky above. This can create the illusion of water on roads, or objects floating in the air:
- Light from sky is bent down towards viewer.
- Appears as if reflected off ground or water surface.
- Occurs when air layers closer to road are much hotter than above.
Superior mirage
Cold air layers above refract light from a distant object below. This gives the illusion of a floating or elevated image:
- Light from object is bent upwards.
- Viewer sees light as if coming from higher angle.
- Occurs when air is colder higher up than near the ground.
Minimizing optical distortion
Careful control of material selection, temperature and manufacturing quality is needed to minimize distortions caused by refraction. Techniques include:
- Annealing glass and lenses relieves internal stresses that can bend light.
- Matching refractory indexes in optical contacts and coatings.
- Athermalization to prevent changes with temperature.
- Using low dispersion glass types.
- Combining multiple elements to correct aberrations.
With precision design and manufacturing, modern optical systems can minimize deviations caused by refractive effects.
Conclusion
In summary, refraction and dispersion of light occurs due to interactions between light waves and matter. Shorter wavelengths bend the most. The degree of bending also depends on differences in refractive index between materials. Variations in refraction are utilized in many optical applications, but can also cause distortions and mirages. Advanced optics uses combinations of lens materials and elements to counteract refractive effects.