Light and Reflection Can you see me?

14-1 EM waves as we discussed before is the classification that light falls under; White light is all light of the visible spectrum Red, orange, yellow, green, blue, and violet Not all light is visible to the human eye Xrays, microwaves, and radiowaves have the same properties as visible light but we can’t see them. They are still considered EM waves

EM waves

EM waves depend on λ ƒ Classic EM waves are considered oscillating magnetic fields and electric fields Transverse waves (?) Look at pink sheet to determine how wavelength and frequency affect light

c=λƒ V? All EM waves move at the speed of light Speed of light (C) is 3.00 x 108 m/s Barely changes Equation for speed of light c=λƒ

Problem The AM radio band Mr. Kuehn listens to is WCCO 8.30 x 105 Hz and 9.37 MHz for FM. What are the wavelengths for these frequencies?

Problem The brightest light humans can see is 560 nm in wavelength, what is the frequency of this wavelength? Answer 5.4 x 1014 Hz

Light travels as wave fronts

Brightness Brightness decreases by the square of the distance from the source. As you move away from a light source, the light wave front spreads out more and distributes the energy over a greater area. Each area receives less energy when further away

Inverse square law

Reflection

Reflection Light travels through any uniform medium or absence of medium in straight lines If light comes in contact with different medium, some of the light will be absorbed and some reflected. Reflection is a change of direction of light. Even good mirrors can only reflect 90% of light

Types of Reflection Diffused reflection: reflections in many directions off a rough uneven surface Specular Reflection is off a perfect smooth surface and all reflection is in one direction

Incoming and reflected angles Angle of incidence = angle of reflection θ=θ’ Angle of incoming= angle of reflected from Normal

Flat Mirrors When images reflect off of flat mirrors, the object appears to be behind the mirror. The distance the image appears to be behind the mirror, “q” and the objects distance from the flat mirror, “p”, are EQUAL! The image behind the mirror is called the virtual image

Image locations Image locations can be located with ray diagrams Ray diagrams can be drawn to locate images using p, q, θ and θ’.

Steps 1)Draw object and image at respective p and q from a parallel mirror making sure their heights are the same 2) Draw a perpendicular “ray” of light. Once the ray passes through the mirror, make a dashed line. 3) Draw an angled “ray” including θ measurements. Draw the same “ray” for the image.

Flat Mirrors p = q θ=θ’ h=h’ Left Right reversal happens

Curved Mirrors

Curved Mirrors Curved Mirrors reflect light too Concave Mirror Convex Mirror Factors that affect image: Amount of curve =radius of curve “r” Center of curvature “c” Draw light left to right.

Real or virtual REAL- It usually appears INVERTED (depends on the type of mirror e.g convex, concave) It can be OBTAINED ON A SCREEN In case of mirror, the image lies in front of the reflecting surface. In case of lens, the image lies on the other side of the object. The light rays meet at a focal point in front of the mirror VIRTUAL- It usually appears ERECT It CANNOT be obtained on a screen In case of a mirror, the image lies behind the mirror In case of lens, the image lies on the same side of the object The light rays meet at a focal point behind the mirror

Concave Real images are images formed when rays of light actually intersect at a single point To find image location 1/p + 1/q = 2/R p = object distance q= image distance R= radius of curvature 1/p + 1/q = 1/f F= focal length

Magnification Magnification for mirrors M=h’/h = -q/p Signs =image height/ object height = -image distance/object distance M=h’/h = -q/p Signs + for upright images virtual images - for inverted real images

Ray diagrams for mirrors Rays Object to mirror Mirror to image 1 Parallel to axis Through F 2 3 Through Center of curvature “C” Back along itself through “C”

Drawing

Example A concave mirror has a focal length of 10.0 cm. Locate the image of a pencil that is place upright 30.0 cm from the mirror. Find the magnification of the images. Draw a ray diagram q = 15 cm M= -.50

Example 2 If you had a large concave telescope with a focal length of 2.5 m. Where would you place an object in front of the mirror in order to form an image at a distance of 3.75 m? What would the magnification be? If the image height were 6.0 cm, what would the object height be? P= 7.50 m M= -0.500 h= 12 cm

Convex Mirrors Convex mirrors curve out. Same rules apply Rays Object to mirror Mirror to image 1 Parallel to axis Through F 2 3 Through Center of curvature “C” Back along itself through “C”

Symbol Situation sign p Object in front of mirror + q Image in front of mirror (real) Image is behind mirror (virtual) - R,f C in front (convex) C in back (concave) Flat mirror infinite h' Image above axis Image is below

Example 539 An upright object is placed in front of a convex mirror with a focal length of 8.00 cm. An upright image 2.5 cm tall is formed 4.44 cm behind mirror. Find the position of the object, magnification and height of the object. P=10.0 cm M= 0.444 H=5.63 cm

Example 2 The largest jelly fish every caught has tentacles 36 m long. Supposed the jelly fish is located in front of a convex mirror 36.0 m away. If the mirror had a focal length of 12.0 m, how far from the mirror is the image? What is the h’? -9.001 m M= .250 H’= 9.001