Reflection and Mirrors

Reflection

Reflection: The bouncing back of a particle or wave that strikes the boundary b/w 2 media. Waves reflect in straight lines, like a particle Ex: Mirrors reflect EM waves, echoes reflect

sound waves

The Law of Reflection

• Incident (incoming) rays and reflected (outgoing) rays make equal angles along with a perpendicular to the surface called the normal.

• The angle made by the incoming ray and the normal is called the angle of incidence.

• The angle made by the reflected ray and the normal is called the angle of reflection.

Angle of Incidence = Angle of Reflection

Check Your UnderstandingCheck Your Understanding

If the angle of incidence is 30 degrees, what will the angle of reflection be on a plane mirror?

30 degrees.

Will it be more or less on a curved mirror?

The same The angle is based on the normal so the angle of incidence ALWAYS equals the angle of reflection, no matter what the same of the mirror is.

Mirrors

• The number of rays which strike a mirror are infinite and every one obeys the law of reflection.

• There are 3 basic shapes to mirrors:1.Plane

• Flat

2.Convex• Thicker in the middle

3.Concave• Thinner in the middle (“caves in on itself”)

Image Formation

Virtual image: an image from reflection or refraction formed from diverging light rays.

An image that CANNOT be projected on a screen

Light does not actually focus there Ex: Flat mirror, convex mirror, diverging lens

Real image: an image from reflection or refraction formed from converging light rays. An image that CAN be projected on a screen Light actually focuses at this point Ex: plane lens, concave mirror, converging

lens

Plane Mirrors

• For objects in a plane (flat) mirror, object image size and object distance equals image distance. Create only virtual images If an object is 1 m in front of a plane mirror, its

image will appear 1 m behind the mirror at the same height.

Check Your Understanding

If you wish to take a picture of your image while standing 2m in front of a plane mirror, for what distance should you set your camera to provide sharpest focus?

4 m. Since you are 2m in front of the mirror, your virtual image will appear 2m behind the mirror, making the sum 4 m.

Focal Point

• The focal point is the point where light converges.

• In a concave mirror, parallel light rays converge (focus) into one point.

• In a convex mirror, parallel light rays diverge (spread out) away from one point

• The virtual image formed by a convex mirror, the virtual image is smaller than the object is.

• When the object is close to a concave mirror, the virtual image is larger the object is.

Mirror Image Formation

Mirror Outside the focal point

Inside the focal point

Convex Virtual

Upright

Smaller

Virtual

Upright

Smaller

Concave Real

Inverted

Smaller

Virtual

Upright

Larger

Check Your UnderstandingCheck Your Understanding

Why will a plane mirror never produce a real image?

The light rays never converge into one point, so the image cannot be projected onto a screen.

What type of mirror must an overhead projector use?

A concave mirror. It is the only type of mirror able to create a real image.

Check Your UnderstandingCheck Your Understanding

Which type of mirror is used on the passenger side mirror on a car (hint: it says on the mirror “Objects are closer than they actually appear”)?

A convex mirror. The image is upright and smaller, giving a wider peripheral view. This is also why they are used in convenience stores and drive thrus.

Check Your UnderstandingCheck Your Understanding

What type of image is produced by a make-up mirror up close?

A virtual, upright, enlarged image.

What type of mirror is a make-up mirror?A concave mirror. Shaving mirrors are the

same type of mirror b/c they also produced enlarged images.

Diffuse Reflection

• Diffuse reflection occurs when light is incident on a rough surface and is reflected in many directions.

Reflection of Sound

• An echo is reflected sound.

• Acoustics is the study of the reflective properties of sound.

• When the walls of a room, auditorium, or concert hall are too reflective, the sound becomes garbled. This is due to multiple reflections called reverberations.

• When the reflective surfaces are more absorbent, the sound level is lower, and the hall sounds dull and lifeless.

• The walls of concert halls are often designed with grooves so the sound waves are diffused. In this way a person in the audience receives a small amount of reflected sound from many parts of the wall, rather than a larger amount of sound from one part of the wall.

• Highly reflective surfaces are often placed behind or above the stage to direct sound out to the audience.