# 有關物理學問題

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**...=causes of why medium has its affect on the speed

Refraction is the change in direction of a wave when it moves into a new medium which causes it to travel at a different speed*. A wave which has undergone refraction is called a refracted wave. Its wavelength* increases or decreases* with the change in speed, but there is no change in frequency.

**Its wavelength.

**The amount of which has refracted.

When light rays move into a new medium, they are refracted according to the laws of refraction of light*. The direction in which they are refracted depends on whether they move into a denser or less medium*(Thus speed changes during this process) and are consequently slowed down or speeded up.

**Its density

Laws of refraction of light:

1.The refracted ray lies in the same plane as the incident ray and normal at the point of incidence.

2.The ratio of the sine of the angle of incidence* to the sine of the angle of refraction* is a constant for two given media.

2008-12-31 20:07:48 補充：

If second medium(for an example, you have a glass on a table, the liquid inside is the second medium) is denser than the first medium(outside the liquid), the light ray slows down and is refracted toward normal. If less dense, ray speeds up and is refracted away from normal.

2008-12-31 20:08:01 補充：

Here normal means line at right angles to boundary through chosen point, eg: point of incidence, point at which incident ray meets boundary(the point which is refracted) and becomes refracted ray.

Hope this would be useful for you :-)

2009-01-01 13:21:33 補充：

Suppose you have a cup of water. So you have air(less dense) and water(denser). When the light waves enter in the water from* the air, the waves slow down because the water is denser than the air(from air to water).

2009-01-01 13:21:37 補充：

This can be reversed respectively. The wave speeds up on entering a less dense medium(from water to air)

2009-01-01 13:27:08 補充：

Regardless to the above, lens can affect the light waves speed as well. By calculating the power of lens: P= 1/f. The shorter the focal length(f), the more powerful the lens. Thus speed up more.

(Note: All regard to the laws of refraction of light)

2009-01-01 20:59:05 補充：

Conclusion: The density of a medium changes the light wave speed

2009-01-01 21:00:43 補充：

Side affects: The amount of which has absorbed can slow the speed down as well

Source(s): , Illustrated Dictionary of Science, myself, teacher, Illustrated Dictionary of Science, myself, teacher
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Light is electromagnetic wave (EM wave) in nature. It travels in space by the mutual interaction of electric and magnetic field components.

When light (EM wave) enters a medium, the electric field and magnetic field components interact with the molecules of that medium, which are held together by intermolecular force that is an electromagnetic force.

It has been shown, by the famous physicist James Clark Maxwell in the 19th Century, that the speed of light (EM wave) in a substance is determined by the permitivity (this is a term to describe the electric property of a substance) and permeability (this is a term describing the magnetic property of the substance) of the substance. Maxwells derivation showed that the higher the permitivity or permeability of a substance, the slower the EM wave travels in that substance. Since vacuum has the lowest permitivity and permeability, thus light travels fastest in vacuum than in other substances.

For most substances, values of their permeability do not varies much within the optical frequency range. Thus, the speed of light in these substances only depends on the value of permitivity.

That is to say, the electric component of the light wave sets the molecules of the medium into vibration Such process absorbs energy from the light wave and the energy is re-emitted again by the vibrating molecules. This absorption and re-emission process takes time (although very very short), so that the wave seems to travel slower in the medium.

Also, the absorption and re-emission prcess depends on the molecular structure of the medium (i.e. how hard the molecules are bound together) . Therefore, the speed of light in the medium depends on the nature of the medium substance.