L 32 Light and Optics 2 Measurements of

• Slides: 32

L 32 Light and Optics [2] • Measurements of the speed of light • The bending of light – refraction • Total internal reflection • Dispersion • Rainbows • Atmospheric scattering • Blue sky and red sunsets • Mirrors and lenses n

The index of refraction (n) depends of the color (wavelength) of the light color Wavelength (nm) n Red 660 1. 520 orange 610 1. 522 yellow 580 1. 523 green 550 1. 526 blue 470 1. 531 violet 410 1. 538 1 nm = 0. 00001 m

Different colors are refracted (bent) by different amounts White light contains all wavelengths (colors) red Glass prism violet

The rainbow • Rainbows are caused by dispersion of sunlight from water droplets which act as tiny prisms

Why is it a rain BOW ? The drops must be At just the right Angle (42 degrees) Between your eyes And the sun to see The rainbow. This Angle is maintained Along the arc of a Circle.

Atmospheric scattering • Why is the sky blue and sunsets red? • It is due to the way that sunlight is scattered by the atmosphere (N 2 and O 2) • Scattering atoms absorb light energy and re-emit it but not at the same wavelength • Sunlight contains a full range of wavelengths in the visible region

Atmospheric scattering: blue sky • Short wavelengths are scattered more than long wavelengths • Blue light (short) is scattered 10 times more than red light • The light that we see in the sky when not looking directly at the sun is scattered blue light

Atmospheric scattering: red sunset • At sunset, the sun is low on the horizon • When looking at the sun it appears red because much of the blue light is scattered out leaving only the red

Mirrors reflection • Light does not pass thru metals – it is reflected at the surface • Two types of reflection: diffuse and specular Diffuse reflection: Fuzzy or no image specular reflection: Sharp image

The law of reflection • The angle of reflection = angle of incidence • Incident ray, reflected ray and normal all lie in the same plane normal Incident ray mirror i r reflected ray

image formation by plane mirrors The rays appear to originate from the image behind the mirror. Of course, there is no light behind the mirror this is called a virtual image Mirrors appear to make rooms look larger.

You only need a mirror half as tall as you are to see your whole self Homer’s image Homer

The image of your right hand is your left hand AMBULANCE is painted backward so that you see it correctly in your real-view mirror

Spherical or curved mirrors Concave mirror Focus parallel light rays are focused to one point

convex mirror focus parallel rays diverge from a focus behind the mirror

Dish antennas signal from satellite detector at the focal point of the dish

Magnifying mirrors Homer • when something placed within the focus of a concave mirror, an enlarged, upright image is formed. • this principle is used in a shaving or makeup mirror Homer’s image

Convex mirrors: wide angle view Object Image A convex lens provides a wide angle view. Since it sees more, the images are reduced in size. Passenger side mirrors are often of this type with the warning: “objects appear further than they actually are". Because they appear smaller they look further away.

Image formation with lenses • converging lens (positive lens) • diverging lens (negative lens) • the human eye – correcting for nearsightedness – correcting for farsightedness • optical instruments • lenses are relatively simple optical devices • the principle behind the operation of a lens is refraction the bending of light as it passes from air into glass (or plastic)

converging lens focal point F a converging lens focuses parallel rays to a point called the focal point. a thicker lens has a shorter focal length

Diverging lens F A diverging lens causes parallel rays to diverge as if they came from a focal point F

Image formation by a converging lens image object 2 F F If the object is located at a distance ofat least 2 F from the lens, the image is inverted and smaller than the object. The image is called a REAL image since light rays actually converge at the image location

A converging lens is used to focus rays from the sun to a point since the sun is very far from the lens, the rays are nearly parallel

converging lens is used in a camera to focus light onto the film when you focus a camera, you adjust the distance between the lens and the film depending on the object location.

Image formation by a diverging lens Object image The diverging lens produces an image that is upright and diminished in size. It is a VIRTUAL image, since light rays do not actually pass through the image point

a magnifying lens F F Object virtual image By placing the lens close to the object we get a magnified virtual image.

Sight – the human eye • Physics of the human eye • Corrections for abnormal vision • Nearsightedness • Farsightedness

The Eye • light enters through the cornea • the iris controls the amount of light that gets in, a muscle can close it or open it, the iris is the colored part • the lens is filled with a jelly-like substance; the ciliary muscle can change the shape of by changing the focal length, (accommodation) the lens and thus change its focal length is able to focus light onto the retina for objects located at various distances

the physics of the human eye The relaxed eye can easily focus on distant objects. To focus on close objects the lens is squeezed to shorten it’s focal length, making it possible to converge the rays onto the retina. The near point is the distance at which the closest object can be seen clearly. It recedes with age.

When a nearsighted person views a distant object, the lens cannot relax enough to focus at the retina. The rays converge too quickly. The remedy is to place a diverging lens in front of the eye to first diverge the

When a farsighted person tries to focus on a close object the lens cannot be squeezed enough to focus on the retina. W point is behind the retina. The remedy is to place The focus e a converging lens in front of the eye to converge the rays h before they enter the eye.

Pencil in lucite block the top half of the pencil is glued exactly at the position where the image of the bottom half is formed in the block due to refraction at the front surface the bottom of the pencil (its image ) appears closer to the front surface of the block the bottom half of the pencil cannot be seen from the sides of the block because any ray from the bottom of the pencil suffers total internal reflection on the sides of the block. top view