Ch 13 Physics Flashcards
(39 cards)
What is refraction?
Refraction: the bending or change in direction of light when it travels from one medium to another
Explain the wagon analogy with refraction.
Wagon analogy: when a wagon travels from pavement onto sand, the front right wheel slows down, while the front left wheel does not slow down as it is still on the pavement. Thai results in the wagon pivoting about the slower right wheel, causing a change in the wagon’s direction
What are the rules of refraction?
The incident ray, the refracted ray and the normal all lie in the same plane (lay on opposite sides of the line that separate the 2 media)
Light bends towards the normal when the speed of light in the second medium is less than the speed of light in the first medium (second medium is more dense than the first medium)
Light bends away from the normal when the speed light in the second medium is greater (second medium is less dense than the first medium)
Why does a spoon in a glass of water appear bent?
→ Light coming from the part of the spoon below the water’s surface must travel through the water into air
→ The speed of light increases from water to air so if it hits the water-air boundary at an angle, light will bend away from the normal
→ The human brain perceives light to travel in a straight line, so it will project these light rays backwards to a virtual light source behind the real spoon
How is partial refraction and reflection used in a two-way mirror?
→ Partial refraction and reflection is the idea that light can be reflected and refracted at the same time
→ This effect can be enhanced if glass has a special film coating behind it that allows some light to be refracted but a lot of it is reflected.
→ This results in a mirrored surface that you can see through but other cannot
How do we see glass?
→ Glass is transparent but we can still see it because it distorts the light we see around it
→ Some of the light gets reflected off the glass
→ Most of the light gets refracted
→ This distortion of light is what allows us to perceive that there is a substance between the source of light and our eyes
What is the index of refraction? Give the formulas.
→ the ratio of the speed of light in a vacuum to the speed of light in a medium is n = C/V
→ n is the index of refraction
→ c is the speed of light in a vacuum
→ v is the speed of light in a given medium
→ The index of refraction can also be calculated using the angle of incidence and angle of refraction: n = sin(angle of incidence)/sin(angle of refraction)
→ n is the index of refraction
→ i is angle of incidence
→ R is angle of refraction
Explain the process of total internal reflection.
→ Light bends away from the normal when it speeds up at the boundary of two media
→ In this case, the angle of refraction is always larger than the angle of incidence
→ As the angle of incidence increases, so does the angle of refraction until it eventually hits 90 degrees
What is the critical angle and total internal reflection?
→ Critical angle: the angle of incidence that results in an angle of refraction of 90 degrees
→ If you increase the angle of incidence past the critical angle the refracted ray will no longer exist in the medium. Instead it will reflect back onto medium
→ The refracted ray disappears, only a reflected ray is visible. This is called total internal reflection.
What are the two conditions for total internal reflection to occur?
Light is travelling more slowly in the first medium than in the second
The angle of incidence is greater than the critical angle (no refraction occurs; all light is reflected back into the medium)
What is a lens? Give examples.
a lens is an optical device which transmits and refracts light
Ex. telescope, night vision goggles, glasses, microscope, camera, projector
Explain the path of light in a lens
→ In a lens, light is refracted twice. Once when going from air to glass and then again when going from glass back to air
→ We are only concerned with the direction of the incident ray entering the lens and the emergent ray leaving the lens
→ Ray diagrams can be simplified by drawing a dashed line through the center of the lens and showing refraction occurring at this lines
What is a converging lens and how is it shaped?
Convex Lens and thicket in the middle
→ A converging lens is a lens that is thickest in the middle and that causes incident parallel light rays to converge through a single point after refraction
What is the optical center (O)?
→ Optical center (O): the point at the exact center of the lens
What is the principal focus (F)?
→ Principal focus (F): the point on the principal axis of a lens where the light rays parallel to the principal axis converge after reflection
→ Because Light can strike from either side of a lens, every lens actually has two foci. To tell them apart, the focus on the same side as the incident light is called the secondary principal focus (F prime)
Where is each foci located in a converging lens?
secondary principal focus is located on left of lens and principal focus is located on right of lens
What is a diverging lens and how is it shaped?
Concave lens
→ A lens that is thinnest in the middle and that causes incident parallel light rays to spread apart after refraction
How are light rays travelling in a diverging lens?
→ Light rays in a diverging lens do not converge but spread apart
→ If you project these diverging rays backwards, it looks as if they come from a virtual focus
→ This point is now the principal focus
→ The secondary principal focus is now on the other side of the lens where the rays actually diverge
→ Note that F and F prime are equally for apart from the optical center of both a converging and diverging lens
How do you locate images in a converging lens?
A ray parallel to the principal axis is refracted through the principal focus
A ray through the secondary principal focus will refract parallel to the principal axis
A ray through the optical center continues straight through without being refracted
What do you get when an object is beyond F prime in a converging lens?
→ When an object is beyond F prime, we always get a real and inverted image
→ The size and location of the image depend on the location of the object
What happens when an object is placed on F prime in a converging lens?
→ When an object is placed on F prime, you get no clear image
→ The refracted rays are parallel and do not cross to form an image, even when extended backwards
What happens when an object is placed between F prime and the lens in a converging lens?
→ When an object is between F prime and the lens, you get an upright, virtual image
→ The virtual image is described as being behind the lens because light rays do not actually arrive at the image location, they only appear to
How do you locate images in a diverging lens?
A ray parallel to the principal axis is refracted as if it had come from the principal focus (F)
A ray that appears to pass through the secondary principal focus (F prime) is refracted parallel to the principal axis
A ray through the optical center continues straight through on its path
→ A diverging lens always produces the same image characteristics no matter where the object is
Give a summary of the SALT charcaterstics in both types of lenses
Converging lens:
Beyond 2F’: smaller, inverted, between F and 2F, real
At 2F’: same size, inverted, 2F, real
Between 2F’ and F’: larger, inverted, beyond 2F, real
At F’: no clear image
Inside F’: larger, upright, behind the lens, virtual
Diverging lens:
Anywhere: smaller, upright, same side as object, virtual