Light and Reflection Chapter 14

Characteristics of Light Section 14.1

Electromagnetic Waves Light is made of electromagnetic waves. Take a prism and break up white light into a rainbow like band of colors. These are all in the visible spectrum. Red, orange, yellow, green, blue, indigo and violet. ROY G BIV

Electromagnetic Waves The spectrum also includes non-visible electromagnetic waves, such as x-rays, microwaves, radio waves, and radiation. Because they all are electromagnetic waves they all have similar properties.

Electromagnetic Waves Electromagnetic waves are transverse waves consisting of oscillating electric and magnetic fields at right angles to each other. Oscillate: to have a periodic vibration

Electromagnetic Waves Electromagnetic waves vary depending on frequency and wavelength All electromagnetic waves move at the speed of light

Electromagnetic Waves We will use 3.00 X 108 m/s as the speed of light, c. The wave speed equation is: c = f Speed of light = frequency X wavelength

Sample Problem The AM radio band extends from 5.4 X 105 Hz to 1.7 X 106 Hz. What are the longest and shortest wavelengths in this frequency range? f 1 = 5.4 x 105 Hz f 2 = 1.7 x 106 Hz c = 3.0 x 108 m/s c = f = c/ f 1 = 5.6 x 102 m 2 = 1.8 x 102m

Light travels in straight lines. Show the laser on the wall. Put an index card in the beam. This shows that the light is traveling in a straight line, but you can only see it when it hits something. Put some chalk dust in the beam to show it is continuous. Brightness decreases by the square of the distance form the source Show how the size of the dot the laser makes gets bigger as it gets further from the source. Laser

The brightness of light is inversely proportional to the square of the distance from the light source. Ex. If you move twice as far away from the light source, ¼ as much light falls on the book.

Flat mirrors Section 14.2

Reflection of Light Reflection – the turning back of an electromagnetic wave at the surface of a substance

Clear vs. Diffuse Reflection Specular reflection: light reflected from smooth shiny surfaces In specular reflection the incoming and reflected angles are equal (=’) Diffuse reflection: light is reflected from a rough textured surface

Part 2 - Reflection Reflection from a mirror: Mirror Normal Incident ray Reflected ray Angle of incidence Angle of reflection Mirror

Reflection of Light Angle of incidence – the angle between a ray that strikes a surface and the normal to that surface at the point of contact. Angle of reflection – the angle formed by the line normal to a surface and the direction in which a reflected ray moves Normal is a line perpendicular to the reflection surface.

Angle of incidence = Angle of reflection The Law of Reflection Angle of incidence = Angle of reflection In other words, light gets reflected from a surface at THE SAME ANGLE it hits it. The same !!!

Drawing a Reflected Image Use ray diagrams to show image location We will find the virtual image (the image formed by light rays that only appear to intersect)

Drawing a Reflected Image Draw the object in front of the mirror Draw a ray perpendicular to the mirror’s surface. Because this is 0 from normal, the angle is the same from the mirror to the virtual object Draw a second ray that is not perpendicular to the mirror’s surface from the same point to the surface of the mirror. Next, trace both reflected rays back to the point from which they appear to have originated, that is, behind the mirror. Use dotted lines when drawing lines that that appear to emerge from behind the mirror. The point at which the dotted lines meet is the image point.

Flat Mirrors Image is VIRTUAL, UPRIGHT, UNMAGNIFIED

Chapter 14 14.3 Concave Mirrors

Concave Spherical Mirrors A spherical mirror has the shape of part of a sphere’s surface. The images formed are different than those of flat mirrors. Concave Spherical Mirror – an inwardly curved, mirrored surface that is a portion of a sphere and that converges incoming light rays.

Concave Spherical Mirrors The light bulb is distance p away from the center of the curvature, C. Light rays leave the light bulb, reflect from the mirror and converge at distance q in front of the mirror. Because the reflected light rays pass through the image point, the image forms in front of the mirror.

Concave Spherical Mirrors If you were to place a sheet of paper at the image point, you would see a clear, focused image of the light bulb (a real image). If the paper was placed in front of or behind the image point, the image would be unfocused.

Concave Spherical Mirrors Real image – an image formed when rays of light actually intersect at a single point Focal length – equal to half the radius of curvature of the mirror.

Concave Spherical Mirrors Mirror equation: 1/p + 1/q = 2/R 1 + 1 = 2 . Object distance Image distance radius of curvature Or: 1/p + 1/q = 1/f 1 + 1 = 1 . Object distance Image distance focal length

Concave Spherical Mirrors Object and image distances have a positive sign when measured from the center of the mirror to any point on the mirror’s front side. Distances for images that form on the backside of the mirror always have a negative sign.

Concave Spherical Mirrors The measure of how large or small the image is with respect to the original object is called the magnification of the image. M = h’/h = -(q/p) Magnification = image height = image distance object height object distance

Concave Spherical Mirrors For spherical mirrors, three reference rays are used to find the image point. The intersection of any two rays locates the image. The third ray should intersect at the same point and can be used to check the diagram.

Rules for drawing reference rays Line drawn from object to mirror Line draw from mirror to image after reflection 1 Parallel to principal axis Through focal point F 2 3 Through center of curvature C Back along itself through C

Ray 1

Ray 2

Ray 3

All three rays together

Spherical Mirrors - Concave F Image is REAL, INVERTED, and DEMAGNIFIED !!!

Concave Spherical Mirror When an object changes its location in relation to the mirror, its image changes in location, and form.

Concave Spherical Mirror Object’s distance Type of Image Location of Image Greater than focal length Real and inverted In front of mirror At the focal length Image is infinitely away from mirror and can’t be seen Between focal point and mirror’s surface Virtual and upright Behind mirror

Distance greater than focal length

Distance = focal length

Between focal length and mirror

Spherical Mirrors – Concave Object Inside the Focal Point Image is VIRTUAL, UPRIGHT, and MAGNIFIED

Concave Spherical Mirrors

M= -(q/p) We have p, but not q, so we need another equation to find q. 1/p + 1/q = 1/f We have p and f, so we can solve for q. 1/q = 1/f – 1/p

1/q = 1/f – 1/p Substitute: (1/10 cm) – (1/30 cm) = 1/q Solve: 0.06667 cm = 1/q q= 15 cm

Now with q we can substitute into the original formula and solve. M= -(q/p) M= -(15 cm/30cm) M= -0.50 This means that the image is smaller than the object and inverted. Therefore it is a real image.

Spherical Mirrors - Convex Convex spherical mirror: An outwardly curved, mirrored surface that is a portion of a sphere and that diverges incoming light rays The focal point and center of curvature are situated behind the mirror.

Spherical Mirrors - Convex Convex mirrors take the objects in a large field of view and produce a small image, but give a the observer a complete view of a large area. Examples: In stores, the passenger’s side of a car

Spherical Mirrors - Convex F Image is VIRTUAL, UPRIGHT, and DEMAGNIFIED

Colour White light is not a single colour; it is made up of a mixture of the seven colours of the rainbow. We can demonstrate this by splitting white light with a prism: This is how rainbows are formed: sunlight is “split up” by raindrops.

Wavelengths of Light Red Light – l = 680 nm Green Light - l = 500 nm Blue Light - l = 470 nm

Adding colours White light can be split up to make separate colours. These colours can be added together again. The primary colours of light are red, blue and green: Adding blue and red makes magenta (purple) Adding blue and green makes cyan (light blue) Adding red and green makes yellow Adding all three makes white again

Only red light is reflected Seeing colour The colour an object appears depends on the colours of light it reflects. For example, a red book only reflects red light: Homework White light Only red light is reflected

(or red and blue, as purple is made up of red and blue): A pair of purple pants, in addition to being ugly, would reflect purple light (or red and blue, as purple is made up of red and blue): Purple light A white hat would reflect all seven colours: White light

Using coloured light If we look at a coloured object in coloured light we see something different. For example, consider the outfit below – I mean, from a physics standpoint, not as a fashion choice: Shirt looks red White light Shorts look blue

In different colours of light this kit would look different: Red light Shirt looks red Shorts look black Shirt looks black Blue light Shorts look blue

Using filters Red Filter Filters can be used to “block” out different colours of light: Red Filter Magenta Filter