Light

Previous knowledge Tell me one of your experience or vocabulary about light.

What is light? Light is _______. Light travels in a _______. Light consists of particles called _________. energy wave photons

Light Spectrum White light can be separated into 6 colors. The lowest frequency of light we can see with our eyes appears ______. The highest frequency of light appear _______. red violet

Electromagnetic Waves(E=hf) What characteristics of light determines the amount of energy carried by a photon of the light? Frequency

E=hf What is the frequency of a photon which has 4.41 x J of energy? (Planck’s constant is 6.63 x kgm 2 /s) E=hf Answer: 6.65 x Hz

Characteristics of light WavesRange of Frequency (Hz)Applications Radio wavesOne millionAM, FM radio and television MicrowavesOne billionRadar, microwave oven Infrared wavesOne trillionHeat, physical therapy Visible lightOne hundred trillionOptical microscopy, optical astronomy Ultra violet10 16 Sunburns, Sterilization of medical instruments X-rays Gamma rays Medical applications Industrial applications

Transparent vs. Opaque Transparent: Materials that allow light to pass through. Ex) water, glass, atmosphere Opaque: Materials that do not allow light to pass through. Ex) wood, stone, metal, people

X-rays Which part of a hand is opaque to x-rays? Skin or bone Which part of a hand is transparent to x- rays? Skin or bone

The Condition of Transparency A chain of absorption and re-emission

Speed of light Light travels at the speed of 3.00 x 10 8 m/s. With this speed, light can travel from the sun to Earth in 8 minutes (about 500 seconds). It takes 4 years for light to travel from the sun to the next nearest star in our galaxy. Our galaxy has a diameter of 100,000 light- years.

v = f λ What is the frequency of a EM wave if it is a wavelength of 1000m? 3.00 x 10 8 m/s = f x 10 5 Hz = f

v = f λ The AM radio station sends a radio wave of 1.0 x 10 6 Hz. What is the wavelength of this wave? 3.0 x 10 8 m/s = 1.0 x 10 6 λ 3.0 x 10 2 m = λ

Light slows down in glass or water. In water light travels at 75% of its speed or 0.75 c.

Refraction or bending of light θ θ’θ’

Wave Interactions Waves can refract, or bend, as they change media.

Index of refraction n = c/v Index of refraction = speed of light in vacuum over speed of light in medium n=1.333 (in water) n=2.419 (in diamond)

Snell’s law n i (sin θ i ) = n r (sin θ r ) Index of refraction of first medium x sine of the angle of incidence = index of refraction of second medium x sine of the angle of refraction θiθi θrθr

Practice of Snell’s Law A light ray of wavelength 589 nm traveling through air strikes a smooth, flat slab of crown glass at an angle of 30.0 degrees to the normal. Find the angle of refraction. (n air =1.00, n glass =1.52) Answer: 19.2 degrees

Critical Angle At the critical angle, light beam no longer emerges into the air above the surface.

The Critical Angle in Sound

Wave Interactions

Distance and light intensity The farther the light is from the source, the more spread out the light becomes. Brightness decreases by a square of distance.

Previous experience How can you look at the back of your head?

Flat mirrors Light always travels in a straight line through a uniform substance. If the surface is opaque, the light is deflected at the surface. The change in the direction of the light is called reflection. Specular reflection: reflected in one direction. Diffuse reflection: reflected in many different directions

Virtual image

Reflection

If a straight line is drawn perpendicular to the reflecting surface at the point when the incoming ray strikes the surface, the angle of incidence and the angle of reflection can be defined with respect to the line.

θ = θ’ Angle of incoming light ray = angle of reflected light ray

Object distance, image distance The relationship between object distance from the mirror which is represented as p, and image distance, which is represented as q, is such that the object and image distance are equal. The image formed by rays that appear to come together at the image point behind the mirror – but never really do – is called a virtual image.

Ray Diagram

p (object distance) = q (image distance) h (object height) = h’ (image height) h pq h’

Draw a ray from the pencil tip perpendicular to the mirror surface. Draw the 2 nd ray from the tip to the mirror at an angle to the surface of the mirror. Then draw the reflected ray. Next, trace both reflected rays back to the point from which they appear to have originated, that is, behind the mirror. By this process, you can locate the complete virtual image of the pencil.

Concave mirrors Concave mirrors focus light to form real images. Real image: an image formed when rays of light actually intersect at a single point.

1/p + 1/q = 2/R Object distance, image distance, and radius of curvature are interdependent. p R f q

1/p + 1/q = 1/f R f 1/object distance + 1/image distance = 1/focal length Light rays that are parallel converge at a single point, which can be represented in a diagram. C F

Magnification relates image and object sizes. The measure of how large or small the image is with respect to the original object’s size is called the magnification of the image. M = h’/h = -q/p M is negative and the image is upside down and real. M is positive and the image is upright and virtual.

Rules for Ray Diagrams RayLine drawn from object to mirrorLine drawn from mirror to image after reflection 1parallel to principal axisthrough focal point F 2 parallel to principal axis 3through center of curvature C back along itself through C

Practice When an object is placed 30.0 cm in front of a concave mirror, a real image is formed 60.0 cm from the mirror’s surface. Find the focal length. Answer: 20.0 cm

Practice A square object is placed 15 cm in front of a concave mirror with a focal length of 25 cm. Find the image distance, magnification, and type of image formed for the object. Draw ray diagrams for the object to confirm your answers. Answer: q = -37 cm, M = 2.5, virtual and upright.

Practice A round object is placed 45 cm in front of a concave mirror with a focal length of 25 cm. Find the image distance, magnification, and type of image formed for the object. Draw ray diagrams for the object to confirm your answers. Answer: q = 56cm, M = -1.2, image is real and upside down.

Polarization for further information.

Linear polarization The process that separate waves with electric-field oscillations in the vertical direction from those in the horizontal direction, producing a beam of light with electric-field waves oriented in the same direction.