# Physics Module 3 - Light and Sound

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Reflection

• Convex = mirror that bends out
• Concave = mirror that bends in
• A normal image can be seen through a flat (plane) mirror.
• An image seen in a mirror is due to reflection of light by the mirror.
• The perependicular line to the mirror is called the normal.
• The angle of incidence is the angle between the incident ray and the normal.
• The angle of reflection is the angle between the reflected ray and the normal.
• Angle of Incidence = Angle of Reflection
• When you look at a mirror image, light rays that reflect off the mirror into your eye appear to come from the image.
• Virtual Image = an image, seen in a mirror (or lens) from which light rays appear to come after being reflected by the mirror (or being refracted by the lens).
• Real Image = an image formed by a lens or concave mirror that can be projected onto a screen.

Curved Mirrors

• When you look closely : an enlarged image of your face.
• Retreat and the image will look blurred.
• Keep moving and the image will appear upside down and very small.
• For a distant point object : the light rays are effectively parallel when they reach the mirror. Then they are focused to the principal focus (focal point), F, of the mirror. A real image is made here.
• The distance from the mirror to the focal point is the focal length.
• For an object beyond the focal point : an inverted real image is formed by the reflected rays. The position and size of the image depends on the distance from the object to the mirror.
• Magnification of the image : image height / object height.
• For an object between the focal point and the mirror : the reflected rays form an upright visual image of the object. The image is magnified behind the mirror.

Convex Mirror

• We use them as rear-view mirrors in cars. The driver has a wider view than a plane mirror. The image of the object viewed in a convex mirror is virtual and smaller than the object.

Refraction

• Refraction = change of direction in which light is passing through.
• In refraction, the light ray :
• - bends towards the normal when it travels from air into glass. the angle of refraction is smaller than the angle of incidence.
• - bends away from the normal when it travels from glass into air. The angle of refraction is greater than the angle of incidence.

Explaining Refraction

• It is a property of all forms of waves including light and sound.
• A glass plate is submerged in a ripple tank. The water above the glass plate is shallower that the water in the rest of the tank. The waves are slower than in deep water.
• They change speed when they cross the boundary, they change direction "
• - toward the normal when they cross from deep -> shallow + slow down
• - away from the normal when they cross from shallow-> deep…