Chapter 23 and 24 Flashcards
Ray model of light
Assumes that light travels in straight line paths (rays). When we see an object, light reaches our eyes from each point on the object. Only a small bundle of rays will enter the pupil of the eye. This model explains reflection, refraction, and formation of images by mirrors and lenses
What happens to light when it strikes the surface of an object?
Some of the light is reflected. The rest is absorbed and transformed to thermal energy, or it is transmitted through (if the object is transparent).
Angle of incidence
The angle an incident ray makes with the normal line, which is perpendicular to the surface, as light strikes the surface
Angle of reflection
The angle the reflected ray makes with the normal
Law of reflection
The angle of incidence is equal to the angle of reflection. The incident and reflected rays also lie in the same plane with the normal to the surface.
Diffuse reflection
When light is incident on a rough surface, it is reflected in many directions. The law of reflection still holds at each small section of the surface. When viewing a normal object, different rays of light will reach your eyes at each angle to the object.
Specular reflection
Reflection from a mirror. When a ray of light shines on a mirror, the light won’t reach your eye unless your eye is positioned at the right place (the spot where the law of reflection is satisfied).
Plane mirror
A mirror with a smooth, flat reflecting surface. Each set of diverging rays that reflect from the mirror and enter the eye appear to come from a single point behind the mirror (the image point). An image point corresponds to each point on the object. Our brain interprets the rays as traveling in straight line paths, even though rays from the object travel to and reflect off of the mirror, and then travel to the eyes from there.
Image distance
The perpendicular distance from mirror to image
Object distance
The perpendicular distance from object to mirror
How are object and image distance related in a plane mirror?
They are equal. The image appears to be as far behind the mirror as the object is in front.
Virtual image
When light rays do not actually pass through the image location itself. The image would not appear on paper placed at the location of the image. Our eyes can see real and virtual images as long as the diverging rays enter our eyes. Light rays don’t pass through the image point, they only appear to diverge from that point
Real image
Light does pass through the image and could appear on a white surface placed at the image position. Light rays pass through and diverge from the image point
Convex spherical mirror
When the reflection takes place on the outer surface of the spherical shape so that the center of the mirror surface bulges out toward the viewer. These mirrors are used on cars because they have a wide field of view.
Concave spherical mirror
If the reflecting surface is on the inner surface of the sphere so that the mirror surface curves away from the viewer (like a cave). These mirrors are used to magnify.
How would the rays reflecting on a spherical mirror behave for an object that was very far away?
The rays would be nearly parallel. They would be exactly parallel if the object was infinitely far away.
Focus
The incident and reflective rays on a spherical mirror form a very small angle with each other. The point where they nearly cross is called the focus.
Principal axis of a spherical mirror
The straight line perpendicular to the curved surface at its center. The incoming rays are parallel to the principle axis in a spherical mirror.
Focal point
The point where incident parallel rays come to a focus after reflection. The focal point is also the image point for an object that is infinitely far away along the principal axis.
Focal length
The distance between the focal point and the center of the mirror
Paraxial rays
Rays that make a small angle with the principal axis- we only examine these. They are near the principal axis and are generally parallel to it.
How are the focal length related to the radius of curvature?
The focal length is half the radius of curvature.
Spherical aberration
Rays only approximately come to a perfect focus at the focal point. The more curved the mirror, the worse the approximation, and the more blurred the image. A parabolic reflector will reflect the rays to a perfect focus- these are difficult and expensive to make, and are used in research telescopes
3 types of rays
- Goes out from the object parallel to the axis and reflects through the focal point
- Goes through the focal point and then reflects back parallel to the axis
- Is perpendicular to the mirror, so the ray reflects back on itself and goes through C (the center of curvature).
When drawing rays, make sure that
The angle of incidence is equal to the angle of reflection
When given an object point, how can you find the image point?
Draw the 3 ray types, the point that all 3 intersect is the image point. If the light passes through the image, it’s a real image
If an object is inside the focal point, its image will be
Behind the mirror
Magnification of a mirror
Defined as the height of the image divided by the height of the object. If m is greater than 2, magnification is occurring.
Sign conventions (2)
- The image height is positive if the image is upright and negative if inverted
- Object distance or image distance is positive if the image or object is in front of the mirror. It will be negative if the image or object is behind the mirror
If an object is outside the center of curvature (c) of a concave mirror, its image will be
Inverted, smaller, and real
Negative magnification indicates
An inverted image
Characteristics of images for a concave mirror
Images can be either real or virtual, and they can be inverted or upright
Characteristics of images for a convex mirror
Images are always virtual and always upright. As the object distance decreases, the virtual image increases in size, yet remains smaller than the object (m is less than 1).
For a convex mirror, the focal point F is on the
Back side of the mirror. Therefore, the focal length and radius of curvature is negative. This is true for all diverging devices.
Refraction
When waves travel into a new medium, their path is usually altered as the speed of the wave changes. The wave will change direction if a ray of light is incident at an angle to the surface other than perpendicular. Refraction is responsible for optical illusions like how objects look underwater.
What happens to fixed vs free ends of a wave during reflection?
The fixed end causes a 180 degree phase shift and the wave becomes inverted. The free end causes no phase shift, so the wave remains upright.
Index of refraction (n)
The ratio of the speed of light in vacuum to its speed in a given material. This value is never less than 1. N can vary with wavelength of light
When light encounters a boundary or a different medium, what happens?
It’s partially reflected and partially refracted. The angle of refraction depends on the angle of incidence and the optical densities (n values) of the two media.
When will a light ray bend toward normal during refraction?
When the index of refraction in the new medium is greater, and the wave slows down.
When will a light ray bend away from normal during refraction?
When the index of refraction is lower, and the wave’s speed increases
Snell’s Law/law of refraction
States that the angle of refraction depends on the speed of light in the two media and on the incident angle. Represented by a formula.
Thermal gradient and refraction of sound
Usually the air nearest the ground is hottest. This creates an upward refraction of sound.
Critical angle
The incident angle at which the angle of refraction would be 90 degrees. There will be a refracted ray at any incident angle less than the critical angle
Total internal reflection
Occurs if the incident angle is greater than the critical angle. This would make the sine of the angle of refraction greater than 1, which can’t happen. Therefore, there is no refracted ray and all of the light is reflected. This only occurs when light strikes a boundary where the medium has a lower index of refraction. Binoculars use total internal reflection
For a concave mirror, how do you determine image characteristics?
Focal length is positive
If image distance is positive, the image is real.
The image is enlarged if the absolute value of the magnification is greater than 1
If magnification is negative, the image is inverted- upright if positive
For a convex mirror, how do you determine image characteristics?
Focal length is negative
Images are always virtual, upright, and reduced in size
Converging lens (positive lens)
A lens that is thicker in the center than at the edges. It makes parallel rays converge to a point and forms a real image. These lenses can produce real (inverted) images or virtual (upright) images depending on the object position
Diverging lens (negative lens)
A lens that is thinner at the center than at the edges- makes parallel light rays diverge. These lenses always produce an upright virtual image regardless of where the object is
How is focal length different between converging and diverging lenses?
Focal length is positive for converging lenses and negative for diverging lenses
For a lens, what determines the sign of the object distance?
The object distance is positive if the object is on the same side of the lens as the light is coming from- it’s negative otherwise
For a lens, what determines the sign of the image distance?
The image distance is positive if the image is on the opposite side of the lens from where the light is coming. The image distance is positive for a real image and negative for a virtual image.
For a lens, what determines the sign of the image height?
The image height is positive if the image is upright and negative if the image is inverted. The object is always upright and positive
How to solve combination of lenses problems
Find the image formed by the first lens like it was alone, then the image becomes an object for the second lens
When the reflection of an object is seen in a flat mirror, the image is
Virtual and upright
When a fish is underwater, where does it appear to be located?
Due to refraction, light from the fish bends away from normal as it exits the surface. This makes the fish appear closer to the surface than it actually is
To shoot a fish with a light beam from a laser gun, you should aim
Directly at the image- the laser is a light beam that will also refract at the surface
If you shine a light through an optical fiber, why does it come out the end but not the sides?
Total internal reflection makes the light reflect from the sides. Since the index of refraction of the fiber is greater than the index of refraction of the surrounding air, the light inside the fiber can totally internally reflect off of the sides. Since the
ends are perpendicular to the sides, the angle of the light relative to the ends is less than the critical angle and the light exits the ends.
Virtual images are formed by
Plane mirrors, curved mirrors, and lenses
You cover half of a lens that is forming an image on a screen. What happens when you cover the top half of the lens vs the bottom half?
The image becomes half as bright in both cases
When happens to an image when an object goes from outside the focal point to inside it, with a converging lens?
The image goes from real and inverted to virtual and upright. The distance of the object from the focal point determines whether it will change in size- it will not change if it moves an equal distance from the focal point on the opposite side
Huygen’s principle
States that every point on a wave front acts as a source of tiny wavelets that spread out in a forward direction at the speed of the wave. The new wave front is tangent to all of the wavelets
Wave front
All points along a wave that form a wave crest
Diffraction
The bending of waves behind obstacles into the “shadow region”. Diffraction occurs for waves but not particles, so we can use this to distinguish the wave nature of light
When is diffraction most prominent?
When the size of an obstacle’s opening is on the order of the wavelength of the wave
Young’s double slit experiment
Light from a single source produced a beam of parallel rays that falls on a screen that contains two small, closely spaced slits. If light consisted of particles, we would expect to see two bright lines on the screen behind the slits. However, the light produced a series of bright lines. This called the wave-interference phenomenon. If the light is a wave, there should be an interference pattern
Constructive interference
When waves from two slits travel the same distance, so they are in phase. A crest of one wave arrives at the same time as the crest of the other wave, so their two amplitudes combine to form a larger amplitude. Can also occur if the waves differ by one wavelength. Produces a bright line during the double slit experiment
Destructive interference
When one ray travels an extra distance of half a wavelength (or a fractional wavelength) the two waves are out of phase. The crest of one wave arrives at the same time as the trough of another, producing zero amplitude. The screen in the double slit experiment will be dark.
Why does interference occur during Young’s double slit experiment?
The interference occurs because each point on the screen is not the same distance from both slits. Depending on the path length difference, the wave can interfere constructively (bright spot) or destructively (dark spot).
Intensity variation (brightness) of interference pattern
The bright fringes are the maximum light intensity, and the dark fringes are minimum light intensity. Intensity is also greatest when m=0 and decreases with increasing orders
What parts of the wavelength contain a spectrum of colors?
Since the position of the maxima (except the central one) depends on wavelength, the first-and higher-order fringes contain a spectrum of colors.
Wavelengths of visible light
400 nm to 750 nm. Shorter wavelengths are ultraviolet; longer are infrared
Dispersion
The spreading of white light into the full spectrum. The index of refraction of a material varies with the wavelength of the light. This is why a prism will split visible light into a rainbow of colors. Actual rainbows are created by dispersion in tiny
drops of water.
Diffraction pattern
Bright and dark fringes around a shadow that exist for any sharp edged object illuminated by a point source.
Fringe width is proportional to
The wavelength of the light and inversely proportional to the slit spacing. Therefore, since light with a longer wavelength and small slit spacing will have the largest fringe width.
The colors in a rainbow are caused by
Different amounts of refraction for light of different colors by the water in the raindrops
Why does a single slit show a diffraction pattern?
There is a path length difference from waves originating at different parts of the slit
If someone is around the corner from you, why can you hear them speaking but can’t see them?
Sound waves have long enough wavelengths to bend around a corner, but light wavelengths are too small to bend much
