Download Week 12-Monday, November 16th

Write down each definition and
match it with the correct term.
1. A picture of an object formed
by light rays
2. The point where parallel light
rays striking a concave mirror
meet
3. A surface that curves out like
the back of a spoon.
Terms: concave, convex, focal
point, optics, image
Possible matching answers
1. A picture of an object formed
by light rays IMAGE
2. The point where parallel light
rays striking a concave mirror
meet FOCAL POINT
3. A surface that curves out like
the back of a spoon. CONVEX
Lenses form images by
refracting light
A medium can refract light
• Refract: the bending of a wave as it
crosses the boundary between two
mediums at an angle other than 90
degrees.
– Mediums: air  lens  air
– Occurs because one side of the light wave
reaches the new medium (lens) slightly
before the other side does.
– That side changes speed, while the other
continues at regular speed, causing it to
turn.
Refraction of light
• Direction in which a light wave
turns depends on whether the
new medium slows the wave
down or allows it to travel
faster.
– Denser medium-slow down
– Thinner medium-speed up
• The normal- imaginary line
perpendicular to the new
surface that helps describe
refraction
– If the medium slows the wave, the
wave will turn toward the normal.
– If the medium lets the wave speed
up, the wave will turn away from
the normal.
Refraction and Rainbows
• Rainbows are cause by
refraction and reflection of
light through spherical water
drops which act as prisms.
– Like a prism, water drops
separate the wavelengths of
sunlight to produce a spectrum.
– Unlike a prism, only one color
reaches your eye from each
drop.
• Red appears at the top of a
rainbow because it is coming
from higher drops
• Violet comes from lower drops
Shape determines how
lenses form images
• Flat mediums- produce a clear image
on other side
• A lens is a clear optical tool that
refracts light- non flat mediums.
– Different lenses refract light in different
ways
– These different images formed are useful
for a variety of purposes
• Types of lenses: convex and concave
Convex Lens
• Curved outward
• Parallel light rays passing through a convex lens are
refracted inward.
– They meet at a focal point on the other side of the lens
– Rays refract twice: when entering the new medium and when
leaving the new medium.
– Rays on the outer part of the lens are refracted the most
– Rays on the principal axis-the line joining the center of the two
curved lenses- will go straight through and not refract.
focal point
principal axis
Concave Lens
Principle Axis
• The lens is curved inward.
• Parallel light rays that pass through a concave lens
are refracted outward.
– Rays do not ever meet.
– Rays refract twice: when entering the new medium and
when leaving the new medium.
– Rays on the outer part of the lens are refracted the most
– Rays on the principal axis-the line joining the center of the
two curved lenses- will go straight through and not refract.
Images formed by lenses
• Type of image depends
on
– Lens type
– For convex lenses: the
distance between the
lens and the object.
• Convex lens: Distance
from lens is measured
in focal length- the
distance from the
center of the lens to the
lens’s focal point.
How Convex Lenses Form
Images
• Look at the 3 steps below to see how an image
is formed by a convex lens. Step 1 is repeated
from the previous slide.
Convex Lenses
Bigger? Smaller? Flip-Flop?
• If the image is more than two focal
lengths away, the image will be upside
down and smaller.
• If the image is between 1 and 2 focal
lengths away the image will be upside
down and larger.
• If an object is less than one focal length
away it will appear right side up and
larger.
Concave Lenses
• Always right side up and smaller than
normal appearance.
• Distance between lens and object does
not make a difference.