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Why does green light slow down more than orange light does when passing through an object?
Question Date: 2015-06-16
Answer 1:

When light hits an object, a few things can happen. Light can partially or wholly reflect, transmit, or absorb. The difference between transmit and absorb is that in transmission, light passes through and is not changed when it comes out the other side of the object, whereas absorption is what gives objects the color we see. This means objects absorb some light and reflect the colors we see, or said another way, the colors not absorbed are what we see.

Your question is related to transmission. Light has wavelike character. Green light has a wavelength of 510 nm (nanometers), and orange is about 590 nm. The speed at which a particular wavelength (color) of light transmits through an object will depend on the nature of the atoms in the object. To explain this, we need to invoke some physics and chemistry. Atoms have electrons. If we imagine the electrons as attached to the atoms with springs, they have the ability to vibrate with a frequency unique to the system. This means that for different atoms and different arrangements of atoms, there will be a different natural frequency for the electrons to vibrate. When light with the same frequency hits the material, the electrons will vibrate in resonance, and the vibration leads to heat generation. In this case where the light and material have the same frequency, the light has been converted to heat energy in the material, meaning light of the frequency that was absorbed will not be transmitted or reflected. On the other hand, transmission and reflection occur when the frequency of light does not match the frequency of the normal vibration of the electrons. In these cases, light hitting the material will lead to vibrations in the electrons, but these vibrations will be brief and light will be reemitted on the same side the light entered (reflection) or reemitted on the opposite side (transmission).

Light in a vacuum travels at about 300,000,000 meters per second, which we call c, the speed of light. When light hits a material, if the frequency of the light does not match the frequency of the material, the light will propagate in the empty space between the atoms at speed c. However, it will eventually interact with an atom in its path, and the wave will set some electrons into vibrational motion, thereby losing some energy. Now the wave is not operating at the original frequency, and as discussed earlier, will be reemitted because the frequency of the wave and the vibrations in the material are not the same. Additionally, this will also slow the speed at which the light is traveling. To answer your question about green vs. orange, it depends on how the atoms are arranged in the material, but can be easily manipulated by utilizing concepts such as polarization. One interesting example is of PLZT flash goggles, whereby the light transmitted is controlled by applying a voltage and tuning the polarization. This manipulation is possible due to the nature of light, more accurately referred to as a type of electromagnetic radiation, where the complex electronic and magnetic effects can be manipulated to create technologies such as the PLZT flash goggles.

Answer 2:

The maximum speed of light is ~3.0x108 m/s in a vacuum (completely empty space). Its speed slows when light passes through a denser medium. The ratio between the speed of light in a medium (v) and in vacuum (c) is called the refractive index (n), where n = c/v. Thus the refractive index of vacuum is 1.0 and all other media is greater than 1.0 – for example, air is 1.0003, water is 1.33, most glass is ~1.5 and lead is 2.6. Denser structures lead to higher refractive indexes: diamond (carbon) has a refractive index of 2.42 but silicon and germanium, which have a similar structure but are denser, have 3.48 and 4.05, respectively.

Regardless of color, all light has the same speed so it should slow down at the same rate (called refraction) when it enters the medium. However, the refractive index of a material can also depend on the wavelength (color) of the light. Most materials have very similar refractive indexes for visible light, so we don’t notice any color splitting as light passes through glass or plastic. Glass prisms are designed to have different refractive indexes at different wavelengths in order to split light into colored bands (called dispersion). For example, violet light might experience n=1.53 in a prism, while red light experiences n=1.51 – this small difference in speed as the light passes through the prism allows the colors become displaced in space to form a rainbow.

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