Optics Mirrors and Lenses

Light Light can be a wave or a particle.Light can be a wave or a particle. Individual particles of light are called photons.Individual particles of light are called photons. Photons have no mass, they are a bundle of energy.Photons have no mass, they are a bundle of energy. The higher the frequency the greater the wave has.The higher the frequency the greater the wave has. UCSD: Physics 8;

Light Intensity Intensity is the the amount of light covering a given surface area.Intensity is the the amount of light covering a given surface area. Intensity is a measure of power, so it is the amount of power over a certain amount of space.Intensity is a measure of power, so it is the amount of power over a certain amount of space. As light gets further away it spreads out more, which means the amount of power on a given area is less and there fore the intensity is less.As light gets further away it spreads out more, which means the amount of power on a given area is less and there fore the intensity is less. –(don’t worry I am not making you do the math here) Spring 2006 UCSD: Physics 8;

Light Intensity Spring 20064

Reflection We describe the path of light as straight-line raysWe describe the path of light as straight-line rays Reflection off a flat surface follows a simple rule:Reflection off a flat surface follows a simple rule: –angle in (incidence) equals angle out (reflection) –angles measured from surface “normal” (perpendicular) surface normal same angle incident ray exit ray reflected ray

Reflection Vocabulary Real Image –Real Image – –Image is made from “real” light rays that converge at a real focal point so the image is REAL –Can be projected onto a screen because light actually passes through the point where the image appears –Always inverted

Reflection Vocabulary Virtual Image–Virtual Image– –“Not Real” because it cannot be projected –Image only seems to be there!

Hall Mirror Useful to think in terms of imagesUseful to think in terms of images “image” you “real” you mirror only needs to be half as high as you are tall. Your image will be twice as far from you as the mirror.

LEFT- RIGHT REVERSAL

Curved mirrors What if the mirror isn’t flat?What if the mirror isn’t flat? –light still follows the same rules, with local surface normal Parabolic mirrors have exact focusParabolic mirrors have exact focus –used in telescopes, backyard satellite dishes, etc. –also forms virtual image

Two types of curved mirrors Convex: Means the mirrors surface bulges outward.Convex: Means the mirrors surface bulges outward. –Rays are reflected away from the center. Spring 2006 UCSD: Physics 8;

Two types of curved mirrors Concave: Means the mirrors surface curves inwards.Concave: Means the mirrors surface curves inwards. –Rays are focused towards a middle point. Spring 2006 UCSD: Physics 8;

Parts of the mirror image We can see an image where the rays meet.We can see an image where the rays meet. This is called a focal point.This is called a focal point. The distance from the mirror surface to the focal point is called the focal length.The distance from the mirror surface to the focal point is called the focal length. For convex mirrors the image is virtual.For convex mirrors the image is virtual. For concave mirrors the image is real.For concave mirrors the image is real. Spring 2006 UCSD: Physics 8;

Curved Mirrors The line running perpendicular through the middle of the mirror is called the principal axis.The line running perpendicular through the middle of the mirror is called the principal axis. If we were to make a complete circle for the mirror the very center of the circle would be at “C” (center of curvature)If we were to make a complete circle for the mirror the very center of the circle would be at “C” (center of curvature) Spring 2006 UCSD: Physics 8;

Objects in mirrors Where you place an object relative to a mirror determines the following:Where you place an object relative to a mirror determines the following: –Is the image real or virtual –Is the image larger or smaller than the true object. –Is the image inverted (up side down) or up right. Spring 2006 UCSD: Physics 8;

For a real object between f and the mirror, a virtual image is formed behind the mirror. The image is upright and larger than the object. For a real object between f and the mirror, a virtual image is formed behind the mirror. The position of the image is found by tracing the reflected rays back behind the mirror to where they meet. The image is upright and larger than the object.

For a real object between f and the mirror, a virtual image is formed behind the mirror. The position of the image is found by tracing the reflected rays back behind the mirror to where they meet. The image is upright and larger than the object. For a real object close to the mirror but outside of the center of curvature, the real image is formed between C and f. The image is inverted and smaller than the object. For a real object between C and f, a real image is formed outside of C. The image is inverted and larger than the object.

For a real object between f and the mirror, a virtual image is formed behind the mirror. The position of the image is found by tracing the reflected rays back behind the mirror to where they meet. The image is upright and larger than the object. For a real object close to the mirror but outside of the center of curvature, the real image is formed between C and f. The image is inverted and smaller than the object. For a real object between C and f, a real image is formed outside of C. The image is inverted and larger than the object. For a real object at C, the real image is formed at C. The image is inverted and the same size as the object. For a real object at C, the real image is formed at C. The image is inverted and the same size as the object.

For a real object between f and the mirror, a virtual image is formed behind the mirror. The position of the image is found by tracing the reflected rays back behind the mirror to where they meet. The image is upright and larger than the object. For a real object close to the mirror but outside of the center of curvature, the real image is formed between C and f. The image is inverted and smaller than the object. For a real object close to the mirror but outside of the center of curvature, the real image is formed between C and f. The image is inverted and smaller than the object.

For a real object at f, no image is formed. The reflected rays are parallel and never converge. For a real object at f, no image is formed. The reflected rays are parallel and never converge.

Mirror Equation Spring 2006 UCSD: Physics 8; We can use math to determine focal length and magnification. Mirror Equation: 1/f = 1/d i + 1/d o f = focal length, d i = distance to image d o = ditsance to object

Magnification Spring 2006 UCSD: Physics 8; You can find out by how much the image was enlarged or Shrunk by simply comparing the two heights. Magnification = h i / h o It also happens to be the same as –d i / d o

Practice Problem Spring 2006 UCSD: Physics 8; An object is placed 12 cm form a mirror and the image appears 20 cm away. What is the focal length? An image appears at a distance of 14m and the mirror has a Focal length of 10m. How far is the object from the mirror?

Refraction Light also goes through some thingsLight also goes through some things –glass, water, eyeball, air The presence of material slows light’s progressThe presence of material slows light’s progress –interactions with electrical properties of atoms. The “light slowing factor” is called the index of refractionThe “light slowing factor” is called the index of refraction Light bends at interface between refractive indicesLight bends at interface between refractive indices –bends more the larger the difference in refractive index n 2 = 1.5 n 1 = 1.0 A B

Convex Lenses Thicker in the center than edges. Thicker in the center than edges. –Lens that converges (brings together) light rays. –Forms real images and virtual images depending on position of the object The Magnifier

Concave Lenses Lenses that are thicker at the edges and thinner in the center.Lenses that are thicker at the edges and thinner in the center. –Diverges light rays –All images are upright and smaller. The De-Magnifier

Lens Equations The lens equation is the exact same as the mirror equation.The lens equation is the exact same as the mirror equation. 1/f = 1/d i + 1/d o1/f = 1/d i + 1/d o The magnification equation is the same too.The magnification equation is the same too. m = h i / h 0m = h i / h 0