IGCSE Unit 2 Light Cambridge IGCSE Physics Adapted by Science Department Great Neck North High School

Theories of Light, Luminous Objects, Ray Model of Light What is light?

What is Light? Wave Theory of Light For most of the 17th, 18th, and 19th centuries, light was understood to travel through space and the Earth’s atmosphere as a wave.

What is Light? Wave Theory of Light, continued… By the end of the 19th century, light was understood to be an electromagnetic wave traveling through space and the Earth’s atmosphere at a constant, very high speed, c (300,000,000 m/s or 186,000 mi/s).

What is Light? Particle Theory of Light In the early 20th century evidence began to accumulate that light traveled as a continuous stream of particles moving at the speed of light, c. These particles came to be known as “photons” and have no mass like ordinary particles (i.e. protons & electrons)

What is Light? Wave-Particle Theory of Light By the mid-20th century, this dual nature of light became accepted. This “wave-particle duality” basically means that light exhibits wave properties at times and particle properties at other times. In other words… light is weird!!! (you can quote me on that)

Luminous Objects Note: Distinguish between luminous (light emitting) objects and light reflecting objects that can be seen because of a luminous object’s light reflecting off of it. Students may need to be reminded they can only see what's in their cloths closet when the light is on and its able to reflect off of their cloths.

Non-Luminous Objects Note: Distinguish between luminous (light emitting) objects and light reflecting objects that can be seen because of a luminous object’s light reflecting off of it. Students may need to be reminded they can only see what's in their cloths closet when the light is on and its able to reflect off of their cloths.

Rays and Beams Light travels in straight lines Rays are represented in diagrams by an arrow Beams are a stream of light that consists of many rays

Beams of Light Arrows must be straight lines Using a straight edge is often necessary Do NOT have to be parallel Note: The two diagrams to the left both represent diverging light rays in a beam of light, the difference is the top one shows the source (luminous object) and the bottom does not. The two diagrams to the right both represent converging light rays in a beam of light, the difference is the top one shows the point where they converge and the bottom does not. Students will use the concepts of converging and diverging light rays later in the unit describing geometric optic lenses.

Beams of Light Light striking a material surface Incident light is either Reflected Bounces off the surface of the material Refracted Transmitted through the material Absorbed Increases the internal energy of the material Warms it up Note: Incident – falling upon or striking a surface.

Light Reflection, Law of Reflection, Protractor Use, Images Formed in Plane Mirrors Reflection of light

Reflection Bouncing off a boundary or surface Particles or matter Atoms Molecules Balls Waves or energy Light Sound Heat Note: Heat is actually infrared light or light that is not visible to the human eye but we will often describe and use them as independent concepts. However most students are probably familiar with luminous objects also producing heat when illuminated. This unit does not require in depth discussion of reflection of matter, sound, or heat specifically but discussing reflection of a bouncing ball could be a useful analogy for student conceptual understanding. If used care must be taken to make sure students can discriminate between reflection of a bouncing ball where a relatively large amount of energy is lost during reflection and reflection of light where relatively little energy is lost.

Protractor Used to measure angles Outer Scale Inner Scale Zero Edge Note: Make sure to emphasize proper placement of the protractor when trying to measure an angle between two segments and how to determine which scale (outer or inner) is appropriate for reporting the angle. Note: Students should be shown an example problem that requires using the straight edge to extend one or both line segment(s) to make measuring the angle correctly easy. Note: Students should carefully examine the protractor they purchase and use, the exact design and location of the features identified can vary. However in good condition all designs should report angles that are very similar to each other. Zero Edge Center Mark or Intersect Point

How to Use a Protractor Activity Measuring Angles Activity Measuring Angles Homework Note: Make copies for students.

Reflection of Light Plane mirror Microscopically smooth and flat surface that reflects light Specular reflection Note: Demo several smooth to the touch materials that show varying degrees of specular reflection. Smooth surfaces showing diffuse reflection are probably available in the classroom (whiteboard, door, posters, etc…). Note: Hash tags on mirror diagram show non reflective surface or back of the mirror. Note: The vocabulary term specular reflection will not be tested.

Reflection of Light Students will be able to use the law of reflection angle of incidence = angle of reflection i = r Note: This could be a good time to introduce the concept of a normal line (imaginary line that is perpendicular or 90o to the objects surface).

Reflection of Light Images Formed by Plane Mirrors Plane mirror image formation Reflects light from the object to the observer Plane mirror image characteristics Virtual image Laterally inverted (inversed) Back to front Same distance behind the mirror as the object is in front Same size, shape, and color Same vertical orientation

Plane Mirror Image Formation Note: The left image is animated. Note: Students can easily determine the location of any part of the objects virtual image using two incident light rays. Note: Students often mistake the image location as being on the surface of the mirror not equal distance behind the mirror.

Virtual Images Virtual images cannot be projected onto a screen The reflected rays appear to come from the mirror image Note: Reflections and holograms are virtual images, they appear to come from a specific location where there is not an actual object located.

Real Images Real images can be projected onto a screen Formed as light rays actually converge on a particular region ALWAYS vertically inverted (∴ upside down) Can be: same size or magnified (enlarged or reduced)

Lateral Image Inversion Note: The object and image are facing each other, the mirror inverted back and front. Closest part to mirror is the closest part of the reflected image. The images left ear is now where the objects right ear is because they are facing opposite directions.

Practice Exam Question Note: Answer His not S! This is a great opportunity to emphasize the importance of reading questions carefully and making sure you are answering it to the best of your ability.

Image Location Note: Students often mistake the image location as being on the surface of the mirror not equal distance behind the mirror.

Line of Sight

Line of Sight

Practice Exam Question Note: The Mirror Practice Exam Question could be given for homework.

Refraction, Dispersion, Total Internal Reflection, & Convex Lenses Refraction of light

Refraction of Light Refraction is the bending of light as it passes from one transparent material into another Occurs for two reasons: (a) because the speed of the light changes; and (b) because the light enters the new material at an angle to the normal Angle of incidence (i) Angle from the normal line that the light strikes the border between the two substances Angle of refraction (r) Angle from the same normal line that the light travels through the new medium Note: Students must be able to correctly discriminate between refraction and reflection in detail. They both use i and r. Key difference is reflected light bounces into the same medium the light was originally in and refracted light travels through a new medium.

Refraction of Light

Refraction of Light Refraction Note: This diagram is a good opportunity to emphasize the difference between the refracted light being shown and what reflected light would do. Refraction

Refraction of Light Demonstration of the refraction of light Object submerged in water Note: This is a good simple qualitative experiment but getting angle of incidence and angle of refraction would be challenging.

Refraction of Light Demonstration of the refraction of light Laser and transparent block Note: This is a good quantitative experiment and measuring the angle of incidence and angle of refraction requires some skill but is not particularly challenging. Students are completing this experiment in their investigating refraction activity.

Refraction of Light As light passes through a parallel-sided transparent material, notice the behavior of the ray… Note: The angle of incidence as the light enters the block equals the angle of refraction as the light leaves the block. These two beams are parallel but do not form a straight line. The angle of refraction as the light enters the block equals the angle of incidence as the light leaves the block.

Total Internal Reflection, Fiber Optics, Lenses, & Dispersion Refraction phenomena

Refraction Phenomenon: Total Internal Reflection When light passes through a transparent material and a fraction of the light is reflected back into that material The remaining light is refracted Total internal reflection When light passes through a transparent material and the inside surface of the material behaves like a mirror, reflecting all the light back into the material Angle of incidence > critical angle Angle of incidence = angle of reflection

Total Internal Reflection The “Critical Angle” The angle of incidence which causes the angle of refraction to be 90o *Can only be reached when light is passing from a slower (optically denser) material to a faster (optically less dense) material Note: The faded purple line represents a weak reflected ray but does not relate to the critical angle concept directly. Once the angle of incidence is greater than the critical angle total internal reflection will occur. Notice the refracted ray moves along the edge of the material

Total Internal Reflection Note: The amount of internal reflection gradually increases as the angle of incidence increases until total internal reflection occurs. total internal reflection

Total Internal Reflection

Total Internal Reflection: Practice Exam Question Note: i1 = refraction, i2 = reflection (total or internal), ii = a refracted ray, toward the normal by eye Note: Refraction of Light Practice Exam Question could be given as homework.

Application of Total Internal Reflection: Fiber Optics

Application of Total Internal Reflection: Fiber Optics Utilizes the phenomenon of total internal reflection Rays of laser light enter one end of the fiber which is optically slower (denser) than air. The rays strike the edge of the fiber at angles greater than the critical angle, thus they are totally internally reflected

Application of Total Internal Reflection: Fiber Optics How are they made? Layer-1 is a protective layer, usually made of some form of plastic to physically protect the inner glass layers Layers 2 & 3 are both glass, however, with different optical densities (light speeds)

Application of Total Internal Reflection: Periscopes There are two ways to make a periscope Reflection with plane mirrors Total internal reflection with two triangular blocks of glass This uses two blocks of glass where light rays are incident greater than the critical angle This version uses two plane mirrors Eye Eye

Refraction Phenomenon: Dispersion of Light (Prism Effect) Note: Students could be introduced to light having different wavelengths and frequencies at this point but they will not be responsible for understanding these concepts until later units.

Dispersion of Light “White” light Visible light that is composed of the full range of visible colors (wavelengths) White light can be dispersed (separated) by most transparent substances Dispersion is separation of white light into the range of visible colors by passing through a transparent substance Occurs because different wavelengths of light are refracted at slightly different angles Note: Red wavelength experiences smallest refraction and violet experiences greatest refraction.

Practice Exam Question Do not answer C, requires understanding of the electromagnetic spectrum which will be covered in the waves unit.

Optical Lenses Curved pieces of glass that refract light for the purpose of forming clear “REAL” images

Thin Converging Lens “Converging” lenses are “Convex” Causes parallel rays of light (from a distant object) passing through the lens to refract and cross the principal axis at a fixed point known as the focal point (F or f) Note: The intensity of the focused light is greatest at the focal point. Taking a magnifying glass and focusing the sunlight on the ground produces a bright spot, moving the magnifying glass toward and away from the ground until you see the brightest spot possible can be used to determine the focal point.

Thin Converging Lens Key Terms: Principal focus (focal point) Point where light rays intersect the principal axis Refracted light rays are focused at this point Focal length Distance from the center of the lens to the principal focus

Thin Converging Lens Note: Converging lens is thicker in the middle than along its edge. A.K.A. Convex lens Note: Students must be aware that the image is not always located at the focal point, the image is only here when the object is relatively far away from the lens. This will be clearer when they learn how to determine image size and location using ray diagrams for converging lenses.

Thin Converging Lens Burning a hole in paper, or starting a fire using a magnifying glass (converging lens) Lens forms a REAL image of the sun at the focal point

Thin Converging Lens How to geometrically show real image formation by a single converging lens… There are three rays that can be drawn to determine (a) location, (b) size, and (c)orientation of the image Line traveling parallel to the principal axis and is refracted so that it passes through the principal focal point on the other side of the lens Line travels through the principal focal point on the same side of the lens as the object and is refracted by the lens so that it travels parallel to the principal axis on the other side of the lens Line travels through the center of the lens (origin) and continues in a straight line on the other side of the lens Note: Origin describes the point where the center of the lens and the principal axis meet. Note: This will be much easier to understand after practicing the skill several times. Note: There is

Thin Converging Lens Note: Make copies of the Practice Converging Lens Ray Diagrams worksheet for students to complete while showing this problem on the SMART Board.

Thin Converging Lens Note: A straight edge is very important for accurate ray diagrams. Note: The object is two focal lengths away from the lens. This is a real image although it has been inverted upside down it is the same size as the object. Note: Our eyes are converging lenses which means our brains sees the world upside down but through development considers that normal and is disorientated when we are upside down and the image it sees is right side up.

Thin Converging Lens Note: The object is less than two focal lengths away from the lens. The real image is further away, inverted (upside down), and magnified. Note: An object at one focal length from the lens will not produce an image. Note: An object less than one focal length from the lens will produce a virtual image on the same side of the lens because the rays will diverge on the other side of the lens but this is beyond the core curriculum and students do not have to be able to draw these ray diagrams. Perhaps it would be good to include in future years.