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Physics 4CChapter 34: Images

Real and Virtual ImagesSpecular and Diffuse ReflectionPlane MirrorsSpherical MirrorsRay TracingConcave MirrorsConvex Mirrors

Thin LensesRay TracingConverging LensesDiverging LensesLenses in CombinationThe Human Eye

Real and Virtual Images

⇒ Actually, a light ray is an idealization; it is meant to represent an extremely narrow beam of light.

The Ray Model of Light ⇒ light travels in straight-line paths called light rays.

⇒ When we see an object, according to the ray model, light reaches our eyes from each point on the object.

Real and Virtual Images

⇒ Although light rays leave each point in many different directions, normally only a small bundle of these rays can enter an observer’s eye, as shown in the figure.

⇒ If the person’s head moves to one side, a different bundle of light rays will enter the eye from each point.

Real and Virtual Images

⇒ Your brain always assumes that light traveled in a straight line path in reaching your eyes.

⇒ Because of this, you often see objects in a different location than where they really are. We have already seen one example of this with refraction.

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Real and Virtual Images

⇒ Another example is the image seen from a plane mirror. ⇒ The reflected light looks as though it came from a point behind the mirror called the image point.

Real and Virtual Images

⇒ In a sense, a virtual image only exists within the brain. However, in physics, a virtual image is said to exist at the perceived location.

⇒ This type of image is called a virtual imagebecause the light rays do not actually emanate from the image point.

⇒ A virtual image can never be formed on a surface or projected onto a screen.

Real and Virtual Images

⇒ A real image, such as one formed by a concave mirror, is an image from which light rays actually do emanate.

⇒ A real image can be projected onto a screen and exists in space whether or not someone is there to view it.

Real and Virtual Images

⇒ A mirage is one type of virtual image that arises from the refraction of light.

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Real and Virtual Images

air of uniform density

air near the ground is less dense so the speed of light is slightly greater

Real and Virtual Images

⇒ Light travels slightly faster through warmer air where the density is less.

⇒ Because of this, light bends towards cooler air.

Specular Reflection

⇒ Reflection from a smooth surface is called specular reflection.

Diffuse Reflection

⇒ Reflection from a rough surface is called diffuse reflection.

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Specular and Diffuse Reflection

⇒ Almost everything we see is because of diffuse reflection.

⇒ If only specular reflection occurred, we could only see objects from certain angles.

Plane Mirror

⇒ The image from a plane mirror is virtual, upright, the same size as the object, and as far behind the mirror as the object is in front of it.

Plane Mirror

⇒ To view one’s full length in a mirror, only a half-length mirror is needed.

Spherical Mirrors

⇒ A spherical mirror has the shape of a section from the surface of a sphere.

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Concave and Convex Mirrors

Concave Mirror

Convex Mirror

Ray Tracing

Ray 1: A ray initially parallel to the central axis will be reflected through the focal point.

Ray Tracing

Ray 2: A ray passing through the focal point will be reflected parallel to the central axis.

Ray Tracing

Ray 3: A ray that travels along a line that passes through the center of curvature C be be reflected upon itself.

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Images from a Concave Mirror

⇒ A object placed between F and the mirror will produce a virtual, enlarged, upright image.

Images from a Concave Mirror

⇒ A object placed between C and F will produce a real, enlarged, inverted image beyond the center of curvature C.

Images from a Concave Mirror

⇒ A object placed beyond C will produce a real, reduced, inverted image between C and F.

Images from a Convex Mirror

⇒ The image produced by a convex mirror is always virtual, reduced, and upright.

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Thin Lenses Thin Lenses

Thin Lenses⇒ If nlens > nsurrounding medium:

Lenses thicker in the middle than at the edges are converging.

Lenses thinner in the middle than at the edges are diverging.

⇒ Ray 1: a ray initially parallel to the central axis is refracted through the focal point on the right side of the lens.

Ray Tracing: Converging Lens

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Ray Tracing: Converging Lens

⇒ Ray 2: a ray through focal point on the left side of the lens is refracted parallel to the central axis.

Ray Tracing: Converging Lens

⇒ Ray 3: a ray directly through the center of the lens is not refracted.

Converging Lens

⇒ An object between F and the lens will create a virtual, upright, and enlarged image.

Converging Lens

⇒ An object between F and 2F will create a real, inverted, and enlargedimage.

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Converging Lens

⇒ An object beyond 2F will create a real, inverted, and reducedimage.

Ray Tracing: Diverging Lens

⇒ Ray 1: a ray initially parallel to the central axis appears to have originated from the focal point on the left side of the lens

Ray Tracing: Diverging Lens

⇒ Ray 2: a ray traveling towards the focal point on the right side of the lens is refracted parallel to the principle axis

Ray Tracing: Diverging Lens

⇒ Ray 3: a ray directly through the center of the lens is not refracted

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Diverging Lens

⇒ A diverging lens always creates a virtual, upright, and reduced image.

Lenses in Combination

⇒ When lenses are used in combination, the image from the first lens becomes the object for the second lens.

The Human Eye⇒ Without a doubt, the human eye is the most remarkable of all optical devices.

The Human Eye

⇒ The fovea is the spot on the center of our field of view where our vision is most distinct. Greater detail can be seen at the fovea that at the side parts of the eye.

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Rods and Cones⇒ There are two kind of “antennae” in the retina for detecting light : rods and cones (named because of their different shapes).

rods ⇒ sensitive to lightness or darkness (but not color)

cones ⇒ sensitive to color (there are three different types of cones)

Rods and Cones

⇒ In the human eye, there are about 6-7 million cones and about 75-150 million rods.

⇒ Cones are concentrated in the fovea. The number of cones decreases as you move away from the fovea.

⇒ Rods are concentrated toward the periphery of the retina.

Rods and Cones⇒ It takes more energy (or more light) to “activate” the color-sensitive cones than it does the rods. ⇒ Because of this, if the intensity of light is very low, the cones will not respond and the things we see will have no color. (This is why stars appear white)⇒ The rods on the periphery of our retina can not see color, but they are very sensitive to motion. Our peripheral vision is poor, but we are sensitive to anything moving in our periphery.

The Human Eye⇒ To see any object, the lens must produce an image of the object at the retina. ⇒ This means that the image distance must always be the same – the distance between the lens and the retina.

⇒ The only way for the image distance i to be the same for all object distances p is for the focal length f of the lens to change.

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The Human Eye⇒ The process in which the lens changes its focal length to focus on objects at different distances is called accommodation.

The Human Eye⇒ The near point is the closest an object can be from the eye and still produce a sharp image on the retina.⇒ For people in their early twenties with normal vision, the near point is about 25 cm. It increases to ~50 cm at age 40 and to ~500 cm at age 60.

⇒ The far point is the farthest an object can be from the eye and still produce a sharp image on the retina.⇒ For people with normal vision, the far point is infinity.

The Human Eye⇒ Nearsightedness (myopia) refers to an eye that can only focus on nearby objects. The far point of the eye is less than infinity.

⇒ Nearsightedness is usually caused by an eyeball that is too long.

The Human Eye

⇒ Nearsightedness can be corrected by a diverging lens that creates a virtual image at the far point of the nearsighted eye.

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The Human Eye⇒ Farsightedness (hyperopia) refers to an eye that can only focus on far away objects. The near point of the eye is greater than 25 cm.

⇒ Farsightedness is usually caused by an eyeball that is too short.

The Human Eye

⇒ Farsightedness can be corrected by a converging lens that creates a virtual image at the near point of the farsighted eye.