Light refraction Chapter 29 in textbook. You’ll want paper for your notes….

Light is a form of energy. Light travels in a straight line Light speed is 3.0 x 10 8 m/s Light is carried by photons Light can travel through a vacuum Light is a transverse wave Light is an electromagnetic wave

ELECTROMAGNETIC WAVES Electromagnetic waves are waves that are capable of traveling through a vacuum. They consist of oscillating electric and magnetic fields with different wavelengths.

The Electromagnetic Spectrum Increasing frequency Increasing wavelength Increasing Energy

The wave speed equation is: c = f λ where c is the speed of light.

When light strikes an object it can be REFLECTED, TRANSMITTED or ABSORBED. Objects are TRANSPARENT, TRANSLUCENT AND OPAQUE to light

Ultraviolet light oscillates at too high a frequency for electrons in glass molecules, while infrared is too low. Visible light is just right. Making glass transparent to visible light and opaque to infrared and ultraviolet

REFRACTION   The bending of a ray of light as it passes from one medium to another is called refraction.

Reflection and Refraction at an Interface

The speed of light c in a material is generally less than the free-space velocity c of 3 x108 m/s. In water light travels about three-fourths of its velocity in air. Light travels about two-thirds as fast in glass. The ratio of the velocity c of light in a vacuum to the velocity v of light in a particular medium is called the index of refraction, n for that material.

Light bends toward the normal when entering medium of higher index of refraction Light bends away from the normal when entering medium of lower index of refraction

SNELL’S LAW The ratio of the sine of the incident angle to the sine of the refracted angle is constant. n1 sinθ1 = n2 sinθ2   n1 = index of refraction of the incident medium n2 = index of refraction of the second medium

Example A ray of light travels from air into liquid Example A ray of light travels from air into liquid. The ray is incident upon the liquid at an angle of 30°. The angle of refraction is 22°. a. What is the index of refraction of the liquid? n1 = 1 1 = 30 2 = 22 n1 sin 1 = n2 sin 2 = 1.33

Critical Angle  n1 sin q1 =  n2 sin q2               =  n2 sin 90     sin q1  =  n2 / n1

= 41˚ Example Find the critical angle for an air-crown glass boundary. ni= 1.52 nr= 1 = 41˚

Light refraction

THIN LENSES  Lenses are an essential part of telescopes, eyeglasses, cameras, microscopes and other optical instruments. A lens is usually made of glass, or transparent plastic.

The two main types of lenses are convex and concave lenses. The focal length (f) of a lens depends on its shape and its index of refraction.

A converging (convex) lens is thick in the center and thin at the edges. A diverging (concave) lens is thin in the center and thick at the edges.

Two types of Images A real image is a representation of an object (source) in which the perceived location is actually a point of convergence of the rays of light that make up the image. A real image is visible on the screen and inverted And formed on the opposite side of a lens..

A virtual image is an image in which the outgoing rays from a point on the object never actually intersect at a point. Virtual images cannot be seen on a screen and form on the same side of a lens.

IMAGE FORMATION BY LENSES   There are three principal rays to locate an image.

Ray 1. A ray parallel to the axis passes through the second focal point F2 of a converging lens or appears to come from the first focal point F1 of a diverging lens.

Ray 2. A ray which passes through the first focal point F1 of a converging lens or proceeds toward the second focal point F2 of a diverging lens is refracted parallel to the lens axis.

Ray 3. A ray through the geometrical center of a lens will not be deviated.

Principal Rays

A real image is always formed on the side of the lens opposite to the object. A virtual image will appear to be on the same side of the lens as the object.

23.7 Find the images formed by the following lenses using the Ray Tracing method. b. Write the characteristics of each image: -real or virtual, -larger, smaller or same size as object and -upright or inverted.

CASE 1: Object is beyond 2F Image is real, reduced, inverted and located between f and 2f on the opposite side of the lens

CASE 2: Object at 2F Image is real, same size, inverted and located at 2f

CASE 3: Object between F and 2F Image is real, magnified, inverted and located beyond 2f

CASE 4: Object is at the focal point No image is formed.

CASE 5: Object is between f and the lens The image is virtual, upright, magnified and located on the same sides as the object

DIVERGING OR CONCAVE LENS Case 1: Object outside of 2f Image is virtual, reduced, upright and located on the same side of the lens as the object

Case 2: Object between f and the lens Image is virtual, reduced, upright and located on the same side of the lens as the object

What if there are TWO lenses?

THE LENS EQUATION   The lens equation can be used to locate the image: Where do is the object’s distance, di is the image distance and f is the focal length. The ratio M is called the magnification, ho is the object’s size and hi is the image size.

23.8 A 5 cm tall object is located 30 cm from a convex lens of 10 cm focal length. a. Find the location and nature of the image. do = 30 cm f = 10 cm = 15 cm, real ho = 5 cm b. What is the height of the image? = - 2.5 cm, inverted

Sign rules R f do di ho hi radius of curvature + converging - diverging   f focal length do object distance + real object di image distance + real images - virtual images ho object size + if upright - if inverted hi image size Sign rules

VISION PROBLEMS: MYOPIA is when image is formed in front of retina and is also known as nearsightedness and is corrected with a concave lens

VISION PROBLEMS: HYPEROPIA is when image is formed behind the retina and is also known as farsightedness and is corrected with a convex lens

VISION PROBLEMS: ASTIGMATISM is when the eye is shaped like a football rather than the normal eye that has a round shape similar to basketball. It causes certain amounts of distortion or pitched images because of the uneven bending of light rays entering the eye.