Which of these lamps is emitting EM radiation?1. Lamp A2. Lamp B3. Both4. Neither

BA

Answer: 3

All bodies with any temperature at all continually emit EM waves. The frequency of these waves varies with temperature. Lamp B is hot enough to emit visible light. Lamp A is cooler, and the radiation it emits is too low in frequency to be visible—it emits infrared waves, which aren’t seen with the eye. You emit waves as well. Even in a completely dark room your waves are there. Your friends may not be able to see you, but a rattlesnake can!

Note that EM waves are everywhere! Not just in air, but in interplanetary “empty space” - actually a dense sea of radiation. Vibrating electrons in sun put out EM waves of frequencies across the whole spectrum. Any body at any temperature other than absolute zero, have electrons that accelerate or vibrate and emit EM radiation that permeates us, even if very low frequency.

A thin beam of light is called a ray.

We see because we have organs (our eyes) that sense the intensity (brightness) and wavelength (color) of light.

If the light is travelling through a uniform medium, light travels in a straight line, and our brain thinks the light is ALWAYS traveling in

the straight line. Our visual systems rely heavily on this fact, 'back-projecting' rays that enter our eyes, to the probable origin of the light rays.

So, if the light has traveled to your eyes in a straight line the object is really where it appears to be. However, if the light entering your eyes has changed the path on the way from origin, your brain will see the object along the extended line entering your eyes.

The eyes do not know laws of physics.

Reflection Mirrors

For now, we are only interested in the light that is reflected at the surface.

The law of reflection

• The angle of reflection = angle of incidence• Incident ray,

reflected ray

and normal all lie

in the same planeIncident

ray

reflectedray

mirror

normal

i r

r i=

How can we get a bunch of parallel rays striking a surface? A beam of light? LASER? YES! BUT THERE IS ANOTHER WAY!!!!If the light source is infinitely far away it is a perfect approximation (example: SUN).

A good approximation is when the source is far enough compared to the dimension of the surface.

=

a small curved mirror

The type of reflection is dependent on the size of the surface irregularities relative to the incident wavelength ().

SPECULAR - 'mirror' reflection Surface particles are small relative to the . Light is reflected in a single direction.

smooth (flat) surface > irregularities in the surface

- bunch of parallel rays encounters an obstacle:

DIFFUSE reflectionSurface is rough relative to the incident . Light is reflected (scattered) in all directions.

Many natural surfaces act as a diffuse reflector to some extent.

rough surface – irregularities<_

All reflections follow the law of reflection

Sharpimage

Fuzzy or no image

IMAGE FORMED BY PLANE MIRROR

An object is in front of a plane mirror. The light is spreading in all directions.

Shown is the path of several rays. This light reflects from the mirror.

The reflected light doesn’t meet (intersect) in the real space, but extended rays behind mirror in the virtual space do. For the eyes it seems as if these reflected light rays were coming from another object back BEHIND the mirror at the intersection of the extended rays !! We call this virtual space because the light never really exists back there....it just SEEMS to be coming from there. We call this apparent source of the light rays a VIRTUAL IMAGE.

Different eyes at different positions; yet - the same image location.

Mirror forms image of every point.

The image is:1. virtual2. the same height (magnification of 1)3. upright (in the same direction)4. equally distant from the mirror as the object

Your eye can’t tell the difference between an object and its image. The light enters your eye the same way it would without the mirror if there really were an object there behind the mirror.

Mirrors appear to make rooms

look larger.

If you measure the length AB you’ll find it will be the half of your height

– the distance from the mirror doesn’t matter!!!

How Large Does A Mirror Need To Be To Show Your Entire body?

A

B

You only need a mirror half as tall as you are to see your whole self

Mr. Stanbrough's Classes

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

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)

The lenses used in optical instruments (eyeglasses, cameras, telescopes, ...) are made from transparent materials that

refract light.

– REMEMBER : LIGHT PASSES THROUGH A LENS

Imagine two prisms and bunch of parallel rays.

Crude lens – two glass prism causing light rays to converge after refraction but they do not converge to one single point

LENSES

Biconvex - Converging lenses

F

Improved lens – parallel beam of rays will converge to a single point on the axis after emerging from the lens (after refraction) .

This point is called focal point F. And that’s exactly the definition of focal point (focus)

F F

thin converging lenses: ignore double refraction in drawings: light ray goes to the middle of the lens and refracts there

FF=

For simplicity: we’ll consider only biconvex symmetrical lens- equal focal lengths ( f )

+ or FF

Optical (Principal) axis

Real focus – parallel rays really meet at focus after refraction through the lens. It is a real image of an infinitely far object.

– REMEMBER : LIGHT PASSES THROUGH A LENSconverging lens

focalpoint F

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

A converging lens is used tofocus rays from the sun to a point

since the sun is veryfar from the lens, therays are nearly parallel

Standard rays to help us draw an image formed by a lens

F F

Converging lenses

F F F F

Image formation by a converging lens

object

image

F2F

If the object is located at a distance beyond 2F from thelens, 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 (to remind you – there are plenty of rays converging there – we drew only two of them

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.

Object: between infinity and 2F p > 2fImage: real, inverted, smaller. That arrangement is used in camera.

object

image

F2F

Object: between 2F & F f < p < 2fImage: real, inverted, enlarged.

Projector

Bulb Object upside-down

Real image

Screen

That arrangement is used in a slide or film projector.

image

Image: virtual, upright, enlarged.

By placing the lens close to the objectwe get a magnified virtual image.

Object: between F & lens p < f

F2F Fobject

The Thin-Lens Equation and the Magnification Equation

f is + for a converging lens

u v f 1 1 1

ih vmh u

object is real: u is + object is virtual: u is – image is real: v is + image is virtual: v is –

In an ideal lens, all light rays from one point of the object would meet at the same point of the image, forming a

clear image. The influences which cause different rays to converge to different points are called aberrations.

Aberrations

Lenses do not form perfect images, and there is always some degree of distortion or aberration introduced by the lens which causes the image to be an imperfect replica of the object. Careful design of the lens system for a particular application ensures that the aberration is minimized. There are several different types of aberration which can affect image quality. (Wikipedia)

object

blurred image

paralel light rays striking the outer edges of a lens are

focused in a slightly different place than beams close to

the axis.

perfect lens spherical lens

Spherical Aberration occurs because spherical surfaces are not the ideal shape with which to make a lens, but they are by far the simplest shape to which glass can be ground and

polished (the least expensive) and so are often used.

This problem is not limited to parallel light. Any incident ray which strikes the outer edges of the lens is subject to this departure from the expected or proper course for the ideal lens. This manifests itself as a blurring of the image. Lenses in which closer-to-ideal, non-spherical surfaces are used are called aspheric lenses.

object

blurred image

cover

object

image

Correction for spherical

aberration

this or money

Chromatic Aberration

This effect can be reduced by having a combination of a convex and a concave lens made of glasses having different refractive indices.

A lens will A lens will notnot focus different colors in exactly the same focus different colors in exactly the same place because the focal length depends on refraction place because the focal length depends on refraction

and the index of refraction for blue light (short and the index of refraction for blue light (short wavelengths) is larger than that of red light (long wavelengths) is larger than that of red light (long

wavelengths). The amount of chromatic aberration wavelengths). The amount of chromatic aberration depends on the dispersion of the glass. depends on the dispersion of the glass.

One way to minimize this aberration is to use One way to minimize this aberration is to use glasses of different dispersion in a doublet or other glasses of different dispersion in a doublet or other combinationcombination

Chromatic aberration can be minimized using additional

lensesIn an Achromat, the second lens cancels the dispersion of the first.

Achromats use two different materials, and one has a negative focal length.

Sight – the human eye

• Physics of the human eye

• Corrections for abnormal vision• Nearsightedness• Farsightedness

• 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 the lens and thus change its focal length

The Eye

by changing the focal length, (accommodation) the lens is able to focus light onto the retina for objects located at various distances

The human eye resembles a camera in its basic structure. Light passé through a lens. A diaphragm, called iris (the colored part of your eye), adjusts automatically to control the amount of light entering the eye. The hole through which light passes (the pupil) is black because no light is reflected from it (it’s a hole), and very little light is reflected back out from the interior of eye. The retina, which plays the role of the film in a camera is on the curved rear surface. It consists of array of nerves and receptors known as rods and cones which act to change light energy into electrical signals that travel along the nerves.

The reconstruction of the image from all these tiny receptors is done mainly in the brain. The sharpest image and the best color discrimination are made at the center of retina, where the cones are very closed packed.

There is no shutter in the eye. The equivalent operation is carried out by the nervous system, which analyzes the signals to form images at the rate of about 30 per second. Movies (US television) operate by taking a series of still pictures at a rate of 24 (30) per second. The rapid projection of these on the screen gives the appearance of motion.

Normal eye (a sort of average) is defined as one having a near point of 25 cm and far point at infinity.

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.

The far point is the farthest distance at which an object can be seen clearly

In nearsightedness, a person can see nearby objects well, but has difficulty seeing distant objects. Objects focus before the retina. This is usually caused by an eye that is too long or a lens system that has too much power to focus.

Myopia is corrected with a negative-focal-length lens (diverging lens). This lens causes the light to diverge slightly before it enters the eye which then converge light at the retina.

eye tends to refract light more than usual

Near-sightedness (myopia)

Far-sightedness (hyperopia) occurs when the focal point is beyond the retina. Such a person can see distant objects well, but has difficulty seeing nearby objects. This is caused by an eye that is too short, or a lens system that has too little focusing power. When a farsighted person tries to focus on a close object the lens cannot be squeezed enough to focus on the retina. Images of closed objects are focused behind the retina, and can not be seen clearly.

Hyperopia is corrected with a positive-focal-length lens (converging lense). The lens slightly converges the light before it enters the eye.

As we age, our lens hardens, so we’re less able to adjust and more likely to experience far-sightedness. Hence “bifocals.”

Far-sightedness (hyperopia)

The optometrists do not specify the focal length of the correctional lenses directly. Instead they use concept of refractive power to describe how much a lens refract the incident light.

Refractive power P of a lens:

unit: diopter (D = m-1)

Pf

1

The size of the image on the retina of the observer depends on

i) the real size of the object, hii) the distance of the object from the observer, u.

The Apparent Size of an Object

These two factors (h and u), determine the size of the angle subtended at the eye by the object.

The apparent size of an object is proportional to this angle. You can see how it changes. The image formed on retina is bigger if the angle is bigger.

Optical instruments provide magnification by increasing the size of the angle subtended at the eye.