WHAT IS REFRACTION? 1.Refraction – bending of light at a boundary between 2 media. a.Optically dense – speed of light in 1 medium is slower than another. b.Willebrord Snell (1621) studied the relationship between the angle of incidence and the angle of refraction (bending).
SNELL’S LAW 1.A ray of light bends in such a way that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant (Snell’s Law). 2.When a ray of light travels from a vacuum to another medium, the formula is: n = sin θ i n is the index of sin θ r refraction (see pg. 490)
SNELL’S LAW CON’T 3.When a ray of light travels from one medium to another medium, the formula is n i sin θ i = n r sin θ r 4.See pg. 490 for “n” values
REFRACTION DIAGRAM refracted ray incident ray Θ r = angle of refraction angle of incidence = Θ i This is the normal medium 1 medium 2
VELOCITY AND REFRACTION 1.It is possible to determine the index of refraction of an unknown object or the speed of light in that object using the following formula n s = c v s
TYPES OF LENSES 1.A lens is a transparent material with a index of refraction > air. a.A lens has 2 faces that are part of a sphere. b.There are two types of lenses – convex and concave.
CONVEX LENSES 1.Convex lens – called converging since it is thicker in the center than at the edges. a. The image produced from this type of lens can be either real or virtual, erect or inverted, enlarged or small. It depends on where the object is with respect to the focal point.
MORE ON CONVEX LENSES b.If the object is located more than twice the distance from the focal point (2F), then the image is real, inverted, and small. c.If the object is located between F and 2F, then the image is real, inverted, and enlarged. d.If the object is in front of the focal point, then the image is virtual, erect, and enlarged.
CONCAVE LENSES 1.Concave lens – called diverging because it is thinner in the middle than at the edges. a.Images produced from this lens are always virtual, erect, and smaller.
USES OF MIRRORS & LENSES 1.Lenses are used to correct defects in the eyes. a.Farsightedness – inability to see near objects so we use a converging (convex) lens. b.Nearsightedness- inability to see far objects so we use a diverging (concave) lens.
MORE USES OF MIRRORS & LENSES 2.Microscopes allow us to see very small objects. a.It uses at least 2 convex lenses. 1.The objective lens has a short focal length which produces a real image located between the eyepiece and the focal length.
STILL MORE USES OF LENSES 3.A telescope is designed to increase the angle between the rays from 2 different stars and to collect more light than the eye would see, not to magnify an image. a. It also uses 2 convex lenses. b. The objective lens has a long focal length which focuses the light rays from a distant object.
STILL MORE USES OF LENSES c.The eyepiece has a short focal length that refracts these rays to produce a virtual, enlarged, inverted image.
TOTAL INTERNAL REFLECTION 1.Occurs when the angle of refraction > the angle of incidence. a.Occurs when a light ray travels from a more optically dense medium to a less optically dense medium. b.The angle that causes the refracted ray to lie right on the boundary is the critical angle.
MORE ON TOTAL INTERNAL REFLECTION c.When the angle of refraction is > than the critical angle, total reflection occurs. d.You can calculate the critical angle using the formula: Sin θ c = 1 where θ c is the n i critical angle
LENS ABERRATIONS 1.All optical instruments use achromatic lenses. These are designed to reduce the “ring of color” that is produced from a single lens. This phenomenon is called chromatic aberration. 2.Spherical mirrors produce spherical aberration which is the inability of a mirror to focus light rays to a single point. This is why parabolic mirrors are used in flashlights, headlights, and search lights.