Optics 光學 1. Reflection of Light

Reflection (反射)

Light rays Light is a source of energy. We detect it with eyes.

Experiment : Reflection by plane mirrors Aims: Study the reflection of light rays by a plane mirror and also find out how to locate the image formed by a plane mirror.

Experiment 1.1 Law of Reflection 1. Direct a ray of light at a plane mirror as shown. Ray-box normal Plane mirror

Experiment 1.1 Law of Reflection 1. Direct a ray of light at a plane mirror as shown. Angle of incidence (入射角) i of the light ray Angle of reflection (反射角) r

Experiment 1.1 Law of Reflection 1. Direct a ray of light at a plane mirror as shown. 2. Measure the angle of incidence and the angle of reflection.

Experiment 1.1 Law of Reflection 1. Direct a ray of light at a plane mirror as shown. 2. Measure the angle of incidence and the angle of reflection. 3. Repeat several times by changing the anlge of incidence.

3. Repeat by changing the angle of incidence. Record the results. Angle of incidence i 0 10 20 30 40 50 Angle of reflection r   How is the angle of incidence related to the angle of reflection? ___________________________________________________________

How is the angle of incidence related to the angle of reflection?

The angle of reflection is equal to the angle of incidence.

Laws of reflection : There are two laws of reflection 1. The incident ray, the reflected ray and the normal all lie in the same plane. This means that all three can be drawn on a flat sheet of paper 2. The angle of incident (i) is equal to the angle of reflection (r). The ray is reflected from the mirror at the same angle as it arrives.

Example 1. (a) What is the angle of incidence? The angle of incidence = 40° Plane mirror (b) Complete the following ray diagram by drawing all the reflected rays. 50° 50° 40° (c) What is the relation between the original incident ray and final reflected ray 40° 50° 50° Plane mirror They are parallel.

Arrange light rays to hit the plane mirror as shown in figure Arrange light rays to hit the plane mirror as shown in figure. Study how the rays are reflected. Look at the rays from above. Also bend down and look along the rays behind The reflected rays seem to come from a point __________ (in front of / behind) the mirror.

reflection abc perpendicular Conclusion: 1. In reflection, the angle of incidence is equal to the angle of _________________.   2. An image formed by a plane mirror is as far behind the mirror as the object is ____________. 3. The line joining the object and the image is _____________ to the mirror. reflection abc perpendicular

Laws of reflection

Object Image

Object Image

Object Image

Complete the rays and located the image Object Image

A man 1.8 m tall stands in front of a plane mirror. His eyes are 1.7 m Above the ground. What is the minimum length of a mirror that allows him to see his whole body? At what height should the mirror be hung? Eye Man Image

A man 1.8 m tall stands in front of a plane mirror. His eyes are 1.7 m Above the ground. What is the minimum length of a mirror that allows him to see his whole body? At what height should the mirror be hung? Eye Man Image

A man 1.8 m tall stands in front of a plane mirror. His eyes are 1.7 m Above the ground. What is the minimum length of a mirror that allows him to see his whole body? At what height should the mirror be hung? Eye Man Image

There are two similar triangles A man 1.8 m tall stands in front of a plane mirror. His eyes are 1.7 m Above the ground. What is the minimum length of a mirror that allows him to see his whole body? At what height should the mirror be hung? Image 0.9m 1.8m (a) Minimum length = 0.9m There are two similar triangles

A man 1.8 m tall stands in front of a plane mirror. His eyes are 1.7 m Above the ground. What is the minimum length of a mirror that allows him to see his whole body? At what height should the mirror be hung? Eye Man Image

A man 1.8 m tall stands in front of a plane mirror. His eyes are 1.7 m Above the ground. What is the minimum length of a mirror that allows him to see his whole body? At what height should the mirror be hung? Eye Man Image

There are two similar triangles A man 1.8 m tall stands in front of a plane mirror. His eyes are 1.7 m Above the ground. What is the minimum length of a mirror that allows him to see his whole body? At what height should the mirror be hung? Eye Man There are two similar triangles (b) It should 0.85 above the ground 1.7m 0.85m

Object

Uses of plane mirrors To help use look at ourselves, To make a room look bigger, In periscopes