Light and Optics

Unit 8: Light and Optics Chapter 22: Optics 22.1 Optics and Reflection 22.2 Refraction 22.3 Mirrors, Lenses, and Images

22.1 Investigation: Reflection Key Question: How do we describe the reflection and refraction of light? Objectives: Identify the range of frequencies humans can hear. Describe the how perception influences the sound humans hear. Make and analyze a histogram of class data.

Optics Optics is the study of how light behaves. Explaining how magnification occurs is part of the science of optics. Diagrams of light use one or more imaginary lines called light rays to show how light travels.

Optics A ray diagram is an accurately-drawn sketch showing how light rays interact with mirrors, lenses, and other optical devices. The curved surface of a magnifying glass bends light so that it appears to come from a much larger thumb.

Reflection and Refraction A lens is an optical device that is used to bend light in a specific way. A converging lens bends light so that the light rays come together to a point. A diverging lens bends light so it spreads light apart instead of coming together.

Reflection and Refraction Mirrors reflect light and allow us to see ourselves. A prism is another optical device that can cause light to change directions. A prism is a solid piece of glass with flat polished surfaces.

Light rays Reflection occurs when light bounces off a surface and when light bends while crossing through materials.

Reflection Images appear in mirrors because of how light is reflected by mirrors. The incident ray follows the light falling onto the mirror. The reflected ray follows the light bouncing off the mirror.

Reflection In specular reflection each incident ray bounces off in a single direction. A surface that is not shiny creates diffuse reflection. In diffuse reflection, a single ray of light scatters into many directions.

The law of reflection The law of reflection states that the angle of incidence equals the angle of reflection. Light rays reflect from a mirror at the same angle at which they arrive.

Law of Reflection The incident ray strikes the mirror. The reflected ray bounces off. The angle of incidence equals the angle of reflection.

Unit 8: Light and Optics Chapter 22: Optics 22.1 Optics and Reflection 22.2 Refraction 22.3 Mirrors, Lenses, and Images

22.2 Investigation: Refraction Key Question: How do we describe the refraction of light? Objectives: Listen to beats and explain how the presence of beats is evidence that sound is a wave. Create interference of sound waves and explain how the interference is evidence for the wave nature of sound.

Refraction A straw in a glass of water makes a good example of refraction. The illusion is caused by refracted light rays when they cross from water back into air before reaching your eyes.

Refraction Materials with a higher index of refraction bend light by a large angle. Refraction occurs when light rays cross a surface between two materials that have a different index of refraction.

What direction does bend light? A light ray going from a low index of refraction into a higher index bends toward the normal line. A light ray going from a high index of refraction to a low index bends away from the normal line.

Refraction Vegetable oil and glass have almost the same index of refraction. If you put a glass rod into a glass cup containing vegetable oil, the rod disappears because light is NOT refracted!

Total internal reflection The angle of incidence at which light begins reflecting back into a refractive material is called the critical angle. Total internal reflection happens when the angle of refraction becomes greater than the critical angle.

Fiber optics A solid glass rod can become a pipe that carries light. This happens if light enters the rod at an angle of incidence greater than the critical angle. Inside the rod, light reflects off of the inside walls and bounces back into the rod because of total internal reflection.

Dispersion and prisms The variation in refractive index with color is called dispersion. A rainbow is an example of dispersion in nature. Tiny rain droplets act as prisms separating the colors in the white light rays from the sun.

Unit 8: Light and Optics Chapter 22: Optics 22.1 Optics and Reflection 22.2 Refraction 22.3 Mirrors, Lenses, and Images

22.3 Investigation: Images from Mirrors and Lenses Key Question: How do mirrors and lenses form images? Objectives: Use the laser flashlight to trace light rays from a lens to determine its focal length. Show how ray diagrams are used to predict where images form with lenses and mirrors. Use the thin lens formula to predict the locations of projected images.

Mirrors, Lenses, and Images We see a world of images created on the retina of the eye by the lens in the front of the eye.

Mirrors, Lenses, and Images Objects are real physical things that give off or reflect light rays. Images are “pictures” of objects that are formed in space where light rays meet.

Mirrors, Lenses, and Images The most common image we see every day is our own reflection in a mirror. The image in a mirror is called a virtual image because the light rays do not actually come together. The virtual image in a flat mirror is created by the eye and brain.

Virtual images Because the light rays do not actually meet, a virtual image cannot be projected onto a screen or on film. Virtual images are illusions created by your eye and brain.

Lenses An ordinary lens is a polished, transparent disc, usually made of glass. The shape of a converging lens is described as being “convex” because the surfaces curve outward.

Mirrors, Lenses, and Images Light rays that enter a converging lens parallel to its axis bend to meet at a point called the focal point. The distance from the center of the lens to the focal point is called the focal length. The optical axis usually goes through the center of the lens.

Converging lenses For a converging lens, the first surface (air to glass) bends light rays toward the normal. At the second surface (glass to air), the rays bend away from the normal line.

Diverging lenses A diverging lens bends the rays outward, away from the focal point.

Drawing ray diagrams A ray diagram is the best way to understand what type of image is formed by a lens, and whether the image is magnified or inverted. These three rays follow the rules for how light rays are bent by the lens: A light ray passing through the center of the lens is not deflected at all (A). A light ray parallel to the axis passes through the far focal point (B). A light ray passing through the near focal point emerges parallel to the axis (C).

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The image formed by a lens A converging lens can also form a real image. In a real image, light rays from the object actually come back together.

Real images To make an image of any object, a lens collects rays from every point on an object. Rays from each point on the object are brought back together again to make each point of the image. Even when you cover half the lens, you still see the whole image.

Magnification The magnification of an image is the ratio of the size of the image divided by the size of the object. A lens with a magnification of 4 creates an image that appears four times larger than the real-life object.

The Telescope When people think of a telescope, most of them think of a refracting telescope. An astronomical refracting telescope is constructed of two converging lenses with different focal lengths. The lens with the longest focal length is called the objective and the shorter-focal-length lens is the eyepiece.

The refracting telescope The image from this refracting telescope is inverted which is usually fine for looking at objects in space.

The reflecting telescope Because high-quality, large lenses are difficult to make, reflecting telescopes use a concave mirror instead of one lens. The diagram shows a reflecting telescope, much like the one used by the Hubble Space Telescope.

Optical systems Optical systems are built from lenses, mirrors, and prisms. Optical systems do two things: collects light rays changes/processes the light rays to form an image. A camera is an optical system that collects light to record an image.

Pin-hole camera A simple optical system can be made with a pinhole in a box. The image inside the box forms because light rays that reach a point on the box surface are restricted by the pinhole to come from only a pinhole-sized point on the object.

Pin-hole camera The larger the lens, the brighter the image. This is because a larger lens collects more light rays. Multiple lenses are useful because they allow an optical system to change the size of an image. A telephoto camera lens uses two or more lenses that move relative to one another.

Recording images There are two basic techniques for recording images. Film records an image by using special inks that respond to light. A digital camera uses a tiny sensor called a CCD.

Recording images There are separate light sensors for red light, blue light, and green light. A color image is recorded as a table of numbers. Each point on the image has three numbers corresponding to the amount of red light, blue light, and green light.

Recording images The resolution of a digital camera is the number of points, called pixels, that can be recorded by the CCD. A 2-megapixel camera stores 2 million pixels per image. Since each pixel is three numbers, a 2-megapixel image requires 6 million stored numbers.

Hope for the Blind Mike, a 28-year-old blind man, lies awake on an operating table, while surgeons place a three- millimeter square electrode panel on the retina of his anesthetized eye. Soon, a medical research team will stimulate the electrodes, in hopes that these electrical impulses will do what the rods and cone cells in his eye once did.