The light spread along a straight line in the same uniform medium。
lunar eclipse 月食
Use a arrow ---------light ray
阴 影 区
2,Speed of light 光的传播要不要时间，光速到底有多快？
1607年，伽利略进行了世界上第一次测量光 速的尝试。方法是，他和助手分别站在相距 1.5千米的两座山上，每人手拿一盏灯。伽利 略先打开灯，当助手看到他的灯光时，立刻 打开自己的灯，从伽利略打开灯至他看见助 手那盏灯的灯光，这个间隔就是光传播了3千 米的时间。
光的速度太大，在相距约1.5km的两 山顶间来回一次，所用的时间大约只 有十万分之一秒，用这种简单粗糙的 方法根本不能测出这样短的时间来。
介质 真空 空气 水 玻璃
C=3×108m/s 稍小于真空中的速度 约空气中的3/4 约空气中的2/3
Objects that reflect light well: ? Have smooth, shiny surfaces and are usually pale colours. ? Give clear images because they reflect light regularly. Objects that do not reflect light well: ? Have rough, matt surfaces and are usually dark colours. ? Give diffuse images (or do not give any images) because they reflect light irregularly. This is called scattering.
Image of a plane mirror
A plane mirror reflects light regularly so it produces a clear image, which is the same size as the object. The image appears the same distance behind the mirror as the object is in front of it. What is different about the image compared to the object? When an object is reflected in a plane mirror, left appears as right and right appears as left. This type of reversal is called lateral inversion.
Reflection ray diagram
normal incident ray reflected ray
angle of incidence
angle of reflection
a plane mirror
Fix a plane mirror to a piece of paper and draw around it.
Draw a normal (a line at 90° to the mirror) through the centre of the mirror outline.
Use a ray box to shine an incident ray at the mirror – plot the incident and reflected rays. Measure the angles of incidence [i] and reflection [r]. Repeat for another four angles of incidence. What do the results show?
angle of angle of incidence [i] reflection [r]
Reflection can be very useful. High-visibility strips are very reflective and make sure that this cyclist gets noticed when there is little light.
How does a periscope use reflection?
The two plane mirrors must be positioned at 45° from the vertical. Light is reflected at right angles from the top mirror onto the bottom mirror and into the eye of the viewer.