Unit 2: Lessons 11 – Partial Refraction and Total Internal Reflection

- Sometimes when you look out a window, you see what

is outside as well as your own reflection

- This is because?

- This phenomenon is called?

- Sometimes when you look out a window, you see what is outside as well as your own reflection

- This is because some light reflects and some light refracts at a surface between two media that have different indices of refraction

- This phenomenon is called partial reflection and

refraction

- Both reflection and refraction occur, but not?

- The amount of each depends on the?

- In this case, more light is refracted than reflected, as shown by the?

- Both reflection and refraction occur, but not equally

- The amount of each depends on the angle

- In this case, more light is refracted than reflected, as shown by the thickness of the rays

Example: Sun shining on water

- If the angle of incidence is nearly zero,

(i.e., sun is directly overhead) what happens?

- As the angle of incidence increases (i.e., at sunset), what happens?

Example: Sun shining on water

- If the angle of incidence is nearly zero, (i.e., sun is directly overhead)

- most of the light penetrates the surface and very little is reflected

- As the angle of incidence increases (i.e., at sunset),

- more light is reflected at the surface and less light penetrates the surface to

be refracted

What feature do most car rearview mirrors have? why?

- A lever that lets the driver choose between 'Daytime' and 'Nighttime' positions

- to control how much light is reflected to the driver's eyes

- and prevent glare from headlights at night.

Shape of rearview mirrors?

- They are wedge-shaped and silvered on the back.

- Light behavior in rearview mirrors

- The light hits at a small angle of incidence,

- gets refracted,

- reaches the silvered back,

- and is reflected to the driver's eyes.

rearview mirrors, Daytime setting?

Light reflected off the back of the mirror is directed to the driver's eyes, allowing a clear view of traffic behind.

Discomfort of Daytime setting at night rearview mirrors?

It reflects the headlights of cars behind directly into the driver's eyes, causing glare.

Nighttime setting rearview mirrors?

Most light penetrates the mirror, refracts, hits the silvered back, and is reflected away from the driver's eyes.

hows does Light reaching driver in Nighttime setting rearview mirrors?

Only a small amount of reflected light reaches the eyes, so the headlights appear dimmer.

Benefit of Nighttime setting rearview mirror?

It reduces glare from headlights while still allowing the driver to see the cars behind.

Q: Why can scuba divers only see objects directly above them on the surface?

A: Because only light coming from directly above penetrates the water and refracts to their eyes. Light at larger angles mostly reflects off the surface instead of entering the water.

Q: What happens to light as the angle of incidence increases at the water's surface?

A: More light reflects away and less refracts into the water, so the diver can't see things outside the narrow cone above.

Q: What does the surface look like to a diver looking up from below?

A: It looks like a bright circular "hole" of light called Snell's window, where light enters the water.

Q: Why can you see stones near you underwater but not ones farther away?

A: Because light from nearby stones can reach your eyes after reflecting and refracting, but light from farther ones reflects away due to refraction limits at the water's surface.

Q: What happens when light travels from water to air?

A: The light bends away from the normal because it's moving from a denser medium (water) to a less dense one (air).

Q: What is the critical angle?

A: The angle of incidence where the refracted ray travels exactly along the boundary (at 90° to the normal).

Q: What happens when the angle of incidence is greater than the critical angle?

A: No refraction occurs — instead, all the light reflects back into the water.

Q: What is the term for when all light is reflected back into the same medium instead of refracting out?

A: Total internal reflection.

Q: How can a glass prism change the direction of light?

A: By creating conditions for total internal reflection, which makes light reflect completely inside the prism instead of refracting out.

Q: What is the critical angle between glass and air, and why is it important for prisms?

A: It's less than 45°. If light inside the prism hits a surface at 45° or more, it undergoes total internal reflection.

Q: In a prism, what are the "short sides" and "long side"?

A:

- Short sides: the two smaller faces forming the right angle, where light enters or exits.

- Long side (hypotenuse): the big slanted face where total internal reflection occurs.

Q: What happens when light enters the prism perpendicular to a short side?

A: It enters straight (0° angle, no refraction), hits the long side at 45°, and is totally internally reflected, changing the light's direction by 90°.

Q: What happens when light enters the long side at an angle?

A: It refracts on entry, reflects 180° inside the prism, and then refracts again when leaving, sending light back in the direction it came from.

Q: What is the rule for total internal reflection in prisms?

A: If the angle of incidence inside the prism > critical angle, all light reflects internally; if it's less, some light refracts out

Q: Why do binoculars use prisms to reflect light t

A: To lengthen the light path inside the binoculars, which is necessary due to the distance to the focal point of the lenses.

Q: What are retroreflectors and where are they used?

A: Small plastic prisms, e.g., bicycle reflectors, that reflect incoming light back 180°, no matter the angle of incidence.

Q: How do retroreflectors work?

A: They use total internal reflection to ensure light is always reflected directly back toward the source, like car headlights.

Fibre Optics

Q: What are optical fibres made of?

Fibre Optics

A: A glass core surrounded by an optical cladding made of a different type of glass.

Q: How does light travel through an optical fibre?

A: Light enters almost parallel to the fibre, hits the core-cladding boundary at angles greater than the critical angle, and undergoes total internal reflection, even around bends.

Q: What are the telecommunications benefits of fibre optics over copper cables?

A:

- Signals are immune to electrical storms

- Can carry more signals over longer distances

- Smaller and lighter than copper cables

Q: How are fibre optics used in medicine?

A: In endoscopes, bundles of fibres:

- Carry light into the body

- Carry video signals back to a monitorThis allows - minimally invasive surgery and faster recovery.