兩個物理問題(ce程度)

1. 紅光在玻璃時的速度為什麼比紫光快?
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 glass medium, the electric field and magnetic field components interact with the glass molecules, held together by intermolecular force which is an electromagnetic force.

It has been shown, by the famous physicist James Clark Maxwell in the 19 Century, that the speed of light (EM wave) in a substance is determined by the permitivity (this is a term in physics to describe the electric property of a substance) and permeability (this is a term in physics describing the magnetic property of the substance) of the substance. Maxwell's 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 vary much within the optical frequency range. Thus, the speed of light in these substances only depends on the value of permitivity.

Values of permitivity for glass depend on wavelength. The longer the wavelength, the smaller is the value of permitivity. Since red light has the longest wavelength in the visible spectrum, the permitivity for red light is the smallest among other colour lights. As the smaller the permitivity, the higher is the speed of EM wave, thus the speed of red light is the highest in glass over other colour lights.


You may refer to the following web-page for more details:

http://en.wikipedia.org/wiki/Refractive_index


2009-04-03 22:02:26 補充：
2.電燈光亮度與什麼成正比?電流/電壓/功率
The brigtness of a bulb depends mostly on the actual power consumed by the bulb. But bulbs of different designs would give different brightness even the same power is consumed by the bulbs. ...(contd)

2009-04-03 22:04:11 補充：
..... This dpends on the mechanism which electrical energy is converted to radiant or luminous energy in the bulb. Bulbs of higher luminous efficacy would give out more light than bulbs of lower efficacy.

[

2009-04-03 22:05:44 補充：
[Luminous efficacy is a measure of the amount of power converted to light, its unit is lumens/watt. Filament bulbs ahve the lowest luminous efficacy, whereas compact flash bulbs have higher luminous efficacy]

1.) 光由紅、橙、黃、綠、青、藍、紫組成，而光的速度在空氣中都是大約3x108m/s，光進入另一媒體時(如玻璃)，因紅光或紫光比其他光先進入玻璃因此光的波長改變了，還未進入媒體的光還是維持原來的波長，因此改變了光的方向。因此，並非紅光或紫光比其他光快，是波長進入玻璃時紅或紫光比其他光先改變了，所以沒有甚麼光比較快這回事。
2.) 電燈的光亮度視乎電燈的構造而定，並非單靠電流或電壓，電流或電壓只是電燈的構造所需，一般而言(或據本人理解)令電燈發光是電燈發光物質所產生，電流或電壓提高相對地制造光的能量亦會增加亦可能增加光亮度(W = V x I)，而電燈中的電阻亦影響其光亮度，因為R = V / I 。