3.2 Light


Ray: The straight line in which light travels. A light ray is represented by a straight line with an arrow to show the direction of motion.
Examples of source of light: Sun, Moon, Lamps, etc.
How do we see an object and the colour of it?

3.2.1 Reflection of light

• Recall and use the law; angle of incidence = angle of reflection

Reflection: Light bounces off on the surface of an object.
Law of reflection.jpg
Image from utexas.edu
Characteristics of an optical image by a plane mirror

• Describe the formation of an optical image by a plane mirror, and give its characteristics
Ray diagram: a diagram that traces the path that light takes in order for a person to view a point on the image of an object.
Supplement
• Recall that the image in a plane mirror is virtual
• Perform simple constructions, measurements and calculations based on reflections in plane mirrors
Virtual image.gif
Ray diagram.gif
Images from physicsclassroom.com
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Ray diagram 2.gif
Optical image by a plane mirror

1. A virtual image; Not a real image. It forms behind the mirror where light does not really come from (Indicate it as dotted lines). The image located behind the mirror is at the same distance from the object to the surface of the mirror.

2. An upright image.

3. Laterally inverted image; Left-right reversal of the image.

4. The image has the same size as the object.

5. The image distance from mirror is the same as the object distance from mirror.

6. The image distance from the object is twice the distance from the mirror to the object.

Practice questions:
1. Light is reflected off the object and the rays strike into the mirror. The rays then are reflected back to our eyes. What is the size of the object in the mirror?
Answer: It's the same as the size of the object.
2. What is the distance from the mirror of the image.
Answer: It is the same as the distance between the mirror and the object. If the distance between the mirror and the object is 5cm, It's also 5cm.
3. To save expenses, you would like to buy the shortest mirror that will allow you to see your entire body. What is the minium lenght of the mirror you can buy?
Answer : The mirror needs to be only half your height.
4. A person 1.80 m tall stands in front of a plane mirror. What is the minimum height of the mirror, and how high must its lower edge be above the floor for the person to be able to see his/her whole body? Person’s eyes are 6.0 cm below the top of the head.
Answer from socratic.org

3.2.2 Refraction of light
• Describe an experimental demonstration of the refraction of light
• Use the terminology for the angle of incidence i and angle of refraction r and describe the passage of light through parallel-sided transparent material

Refraction: Light changes its direction when it passes from one medium to another.

refraction of wavefronts.gif
a_level_physics_notes_snells_law_refraction_total_internal_reflection_critical_angle_html_m2fc87019.gif
Light-Refraction-Glass.gif
Image from http://electron6.phys.utk.edu
Image from www.astarmathsandphysics.com
Image from gcsescience.com


• Give the meaning of critical angle

• Recall and use n = 1/sin c

Critical angle: A special angle of incidence where the angle of refraction is 90 degree.

• Describe internal and total internal reflection
Total internal reflection: The complete reflection of a light ray at the boundary of two media, when the ray is in the medium with greater refractive index

Supplement
• Recall and use the definition of refractive index n in terms of speed
Refractive index: The ratio of the velocity of light in a vacuum to its velocity in a specified medium.
The measure of the bending of a ray of light when passing from one medium into another. If i is the angle of incidence of a ray in vacuum (angle between the incoming ray and the perpendicular to the surface of a medium, called the normal), and r is the angle of refraction (angle between the ray in the medium and the normal), the refractive index n is defined as the ratio of the sine of the angle of incidence to the sine of the angle of refraction; i.e., n = sin i / sin r. Refractive index is also equal to the velocity c of light of a given wavelength in empty space divided by its velocity v in a substance, or n = c/v.

• Recall and use the equation sin i / sin r = n
The laws of refraction with examples from www.physicstutorials.org

Snell’s law
Lab report (20%)

• Describe the action of optical fibres particularly in medicine and communications technology

Total internal reflection occurs when light in a more dense medium reflects at the boundary.

Application of Total Internal Reflection from youtube.com

3.2.3 Thin converging lens
• Describe the action of a thin converging lens on a beam of light
• Use the terms principal focus and focal length

Focal length: The distance from the surface of a lens or mirror to its focal point.

Principal focus: The focal point. The point at which all radiation coming from a single direction and passing through a lens or striking a mirror converges. The point on the axis of a lens or mirror to which parallel rays of light converge or from which they appear to diverge after refraction or reflection

Principal ray: The one ray within a bundle of incident rays that, upon entering an optical instrument from any given point of the object, passes through the optical centre of the lens.

• Draw ray diagrams to illustrate the formation of a real image by a single lens

• Describe the nature of an image using the terms enlarged/same size/diminished and upright/inverted



Mr. Hamann's Ray Diagram Convex lens from youtube.com
Mr. Hamann's Ray Diagram Concave lens from youtube.com

Supplement
• Draw ray diagrams to illustrate the formation of a virtual image by a single lens
• Use and describe the use of a single lens as a magnifying glass
• Show understanding of the terms real image and virtual image

Draw and interpret simple ray diagrams that illustrate the formation of real and virtual images by a single converging and a diverging lens.

Operation of a thin lens applet by Andrew Adams, Nora Willett from Washington.edu
Converging lenses ray diagrams from physicsclassroom.comImage Convex lens.png

Diverging lenses ray diagrams from physicsclassroom.com

Image Concave lens.png
Image formation by a converging lens from
http://en.wikipedia.org/wiki/Lens_%28optics%29

Image formation by a diverging lens from
http://www.colorado.edu/physics/phys1020/phys1020_sp05/labs/Lab4_Optics.htm


Ray diagrams for lenses and mirrors from physicsclassroom.com and hyperphysics.
Practice image from courses.cs.washington.edu
Image Check Points
Convex lens
Concave lens
Position of the image


Real / Virtual


Inverted / Upright


Magnify / Diminish


Same / Opposite side


ray diagrams of a magnifying lens.JPG
Image from www.amherst.edu

3.2.4 Dispersion of light
Core
• Give a qualitative account of the dispersion of light as shown by the action on light of a glass prism including the seven colours of the spectrum in their correct order


Dispersion: The phenomenon that a ray of light splits into its constituent colours (ROYGBIV) when light is passing through a transparent medium. The obtained band of colours due to the dispersion is called spectrum.

White light is made of various colours mixed together. The wavelength of visible light can vary from 400nm to 800nm. If it is passed through a prism, the each wavelength refracts by a different amount and therefore a different angle. It disperse/splits into a band of colour spectrum.

Refraction Through a Prism by tutorVista from youtube.com

Go to S-Cool websites for dispersion and TIR

Supplement
• Recall that light of a single frequency is described as monochromatic

monochromatic: containing or using only one color.
"monochromatic light" - a single wavelength or frequencyof light (Red laser is monochromatic)
(e) Electromagnetic spectrum

What is light? Kurzgesagt – In a Nutshell Published on 15 Oct 2015