Reflection

R E F L E C T I O N

Rightside Up or Upside Down?

Regular Reflection • Law of Reflection

• The angle of incidence always equals the angle of reflection.

• DEB – Sketch an incident ray striking a plane mirror at an angle of 50 degrees. Label the normal, the incident ray, the reflected ray, angle i and angle r.

HITT – What is θ ? A) 35

B) 45

C) 55

D) 60

Regular vs diffuse reflection • In diffuse reflection the rays always obey the law of reflection but since the surface is not smooth, there is no image.

Regular Reflection

Diffuse Reflection

What kind of reflections account for the column of light reflected off the water? What would we see on the water if it were perfectly flat, unmoving?

Plane Mirror • A plane mirror is a piece of glass with silver or aluminum backing. • Draw ray diagrams for a plane mirror

Notice every ray that strikes the mirror obeys THE LAW Notice the virtual source.

The Virtual Image in a Plane Mirror

How Can You Locate an Image in a Plane Mirror ? • DEMO – Locate

the image by • 1. Pointing at it • 2. Parallax • 3. Placing a screen at the image if it is real

Is it Real or Virtual?

Screen Number 1.

Image in a Plane Mirror

Name the actor and win a raffle ticket!

Groucho Marx

Plane Mirror

Plane Mirror • Measure di, do, si, so • di is the distance from the mirror to the

image • do is the distance from the mirror to the object • si is the size of the image

• so is the size of the object

Plane Mirror The size of the image ALWAYS equals the size of the object. si = so The distance to the image ALWAYS equals the distance to the object di = do The image is always VIRTUAL

The girl in Edouard Manet's painting,  The  Bar at theFolies-Bergeres, is standing in front of a large plane mirror. We see reflected in it her back and the face of a man she seems to be talking to. From the law of reflection what if anything, is wrong with this painting?

Concave Mirror • Think of a concave mirror as an infinite number of plane mirrors. Every ray still obeys the law of reflection

Concave Mirrors • f is the distance from the mirror

to the point where parallel rays that go to the mirror all meet. = focal point • Since the light gathers at this point it is called a focal point and this mirror is called a converging mirror • Q –Name a parallel light source? • A – The SUN.

Focal Point

Concave Mirrors • Demo with large concave mirror to start a fire with a parallel light source. • Name some applications -

• Rules for concave mirrors • In parallel – Out through the focal point • In through the focal point – Out parallel • In to the middle of the mirror – Out from the middle of the mirror

Images

• Real images are always inverted and in front of the mirror. They can be projected onto a screen and actually have light rays meeting at the image.

• Virtual images are always erect and are located behind the mirror.

Concave Mirror

• DEMO – Locate the image by • 1. Pointing • 2. Parallax • 3. Placing a screen at the image if it is real

6 Cases for Concave Mirrors • 1• 2• 3• 4• 5• 6-

Object at ∞ Object beyond 2f Object at 2f Object between f and 2f Object at f Object closer than f

Man is closer than the focal point = case 6 Tree is much farther than 2f = case 2

Formulas for Mirrors

• 1/do + 1/di = 1/f • • di/do = si/so • If the image is virtual, di and si are negative.

Convex Mirrors

Convex Mirrors

• There is only one case for a convex mirror. All images are small, erect, and behind the mirror = virtual • Notice the Large Convex Mirror at the front of the room – also called a diverging mirror or a wide angle mirror. • Applications?

Focal point of a convex mirror f is negative – it’s Virtual

Making mirrors better

• Spherical Aberrations -replace spherical mirrors with parabolic mirrors