P5.3 Flashcards
What do lenses do
refract light passing through them.
What is the principle focus
The point where lights parallel rays meet or appear to meet.
What is the focal length
The distance from the lens to the principal focus.
The shorter the focal length….
the more powerful the lens.(because it has refracted the light rays more)
What are convex lenses
Makes parallel rays passing through itconverge (focus) to a point.
Uses of convex lenses
- magnifying lenses
- correcting long - sight
What is a concave lens
Makes parallel rays passing through it diverge (spread out).
Uses of concave lenses
- correcting short-sightedness
How do rays from nearby objects appear
Rays from nearby objects appear to travel diverging from each other
Lens is thick, refracts light rays a lot to bring to focus.
How do rays from distant objects appear to be travelling
Rays from distant objects appear to travel parallel to each other
Lens is thin, refracts light rays a little to bring to focus.
Causes of Short sightedness
Lens is too thick… too powerful… refracts light rays too much… rays come to a focus in front of retina
How do concave lenses fix shirt sightedness
Concave lens diverges rays before they enter eye
A concave lens is used to diverge parallel light rays before they enter the eye.
Causes of long - sightedness
Lens is too thin… too weak… refracts light rays a little… rays come to a focus behind retina
How to convex lenses fix long sightedness
A convex lens is used to converge diverging light rays before they enter the eye.
What is a real image
Image that is formed where light rays meet.
What is a virtual image
Image that is formed where light rays appear to come from.
Which lenses produce real images
object is OUTSIDE the focal length of a converging lens
Which lenses produce virtual image
object is INSIDE the focal length of a converging lens
formed by a diverging lens
What is an upright image
Image is same way up as object.
What is an inverted image
Image is upside down compared to object.
Which lenses produce magnified images
Images appear magnified when an object is placed INSIDE the focal length of a CONVERGING lens.
Equation for magnification
Magnification = image height / object height
Draw a convex lens, with the image OUTSIDE the focal point
What type of image is produced in a convex lens, with the image OUTSIDE the focal point
Image is…
Real
Inverted
Diminished
Draw a convex lens, with the object INSIDE focal point
What type of image is formed from a convex lens, with the object INSIDE focal point
Image is…
Virtual
Upright
Magnified
Draw a concave lens diagram with the object OUTSIDE focal point
What type of images do concave/ diverging lens form
All diverging lens form the same image…
VirtualUpright
Diminished
How to figure out if an image is magnified or diminished from a lens diagram
If the image height is higher then the actual object = magnified
If the image height is lower then the actual object = diminished
Why do thicker lenses have shorter focal lengths
Because light spends more time travelling through the thicker Lens so it will end up being bent more
If a concave lens diverges more, how does that effect focal length
Shorter focal length
If a convex lens converges more, how does that effect focal length
Shorter focal length
What is dispersion
Dispersion is the splitting of white light into its constituent wavelengths (spectrum).
Why does light disperse
Different wavelengths of EM radiation travel at different speeds within glass, so refract by different amounts.
How does wavelength effect refraction
Smaller the wavelength, the more the refraction.
Which refracts more blue or red
Blue
What does it mean if someone sees an object as blue under white light
If an object appears a certain colour under white light (e.g. blue), it is reflecting only that frequency (blue) and absorbing all the other frequencies (red, green, yellow etc.)
IT IS REFLECTING BLUE + ABSORBING ALL OTHER FREQUENCIES
What does specular reflection produce
sharp images(e.g. mirror)
What does diffuse scattering produce
no images(e.g. sheet of paper)
Define a filter
Filters absorb certain frequencies of light but transmit others.
What does a red filter do
will transmit only red light
Are transparent coloured objects filters?
Yes
Polly is wearing a red t-shirt. Jack is wearing a yellow t-shirt.
What colours would their t-shirts appear in…
In red light?
In yellow light?
In blue light?
Red light - polly RED and jack BLACK
Yellow light - polly BLACK and jack YELLOW
Blue light - polly BLACK and Jack BLACK
Blue balloons reflect blue wavelengths of light, under white light.
Red light contains only red light wavelengths.
Thus, when illuminated by red light, the blue balloon will absorb the red light and reflect nothing, appearing black.
Suggest why you do not see a spectrum, when white light goes through rectangular glass block
A rectangular block has two lines of symmetry.
Thus, the final refracted beam travels parallel to the original incident beam.
Therefore, the different wavelengths of light within white light, aren’t separated out.
Shape of concave lens
Shape of convex lens
