The Bending of Light and Lenses

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# The Bending of Light and Lenses - PowerPoint PPT Presentation

The Bending of Light and Lenses. Chapter 18 and 19. Refraction. The bending of light due to a change in speed when switching medium Why a straw appears to break when its in a glass of water Bends towards the normal when the new medium is slower (ex. From air to glass)

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### The Bending of Light and Lenses

Chapter 18 and 19

Refraction
• The bending of light due to a change in speed when switching medium
• Why a straw appears to break when its in a glass of water
• Bends towards the normal when the new medium is slower (ex. From air to glass)
• Bends away from the normal when the new medium is faster (ex. From water to air)
Waves and Refraction
• Since frequency cannot change, but speed does, then wavelength must change as a wave enters a new medium
• Wavelength is less in glass than in a vacuum
• Different frequencies of light refract differently (this is how a prism works)
Total Internal Reflection
• If the angle of refraction is high enough, it becomes impossible for light to escape the medium
• It always gets bent back into it
• This is called the critical angle
• Creates the illusion that all light is reflected back into the medium
• Used in fiber optics (much more efficient than current energy carrying methods)
Mirages
• Hot air near the ground causes light rays to refract back up
• Your eye perceives this as a reflection
• Since there is a “reflection” your mind tells you the ground must be wet, even though it isn’t
Dispersion
• As white light passes through a prism, it is separated into its different colored frequencies
• This is because refraction is caused by how the light interacts with molecules in the glass (or other medium)
• The interactions for violet light are more because it has more energy, therefore it takes longer to get through the glass and refracts more than the red light
• Rainbows are the result of drops of water dispersing light
• Although each drop disperses all the colors, only one color from each drop makes it to your eye
Lenses
• A piece of transparent material that bends light rays to a focus to create an image
• 2 types:
• Concave
• Curve in
• Make virtual images on the same side of lenses as the object (di is negative)
• Convex
• Curve out
• Make real images if object is beyond the focal point (di is positive)
• Make virtual images if object is between focal point and lens (di is negative)
Ray Diagrams
• A diagram to show where and how an image from a lens is formed
• Only need to draw 2 rays: one that passes through the focal point on its way to the lens and one that gets refracted through the focal point on the other side of the lens
Lens Abberations
• Defects caused by looking through lenses
• Spherical aberration
• Slight differences in where light rays focus once they pass through a curved lens
• Chromatic aberration
• Differences in how colors of light are bent differently, resulting in the different colors having different focal points
Interference
• Waves in the same place at the same time
• Can be:
• Incoherent
• All in different phases
• Appears as white light
• Coherent
• Waves combine into smooth wave fronts
• Results in constructive interference and waves with larger amplitudes (more energy)
Double Slit Experiment
• Supported the wave theory of light
• Alternating bands of light are formed when light passes through a double-slit because some undergoes constructive interference and some undergoes destructive interference
Thin-Film Interference
• Occurs because of a combination of reflection and refraction off of multiple surfaces (like a soap bubble or gas on the ground)
• Some frequencies get cancelled and some don’t, resulting in the iridescent (rainbow) effect that you see
Diffraction
• Waves bend for some reason other than a change in medium
• Usually passing through a hole or around a barrier
• Amount of diffraction changes based on wavelength and size of obstacle