Lesson 4. Define the terms principal axis, focal point, focal length and linear magnification as applied to a converging (convex) lens. Define the power of a convex lens and the dioptre. Define linear magnification. - PowerPoint PPT Presentation
Lesson 4 • Define the terms principal axis, focal point, focal length and linear magnification as applied to a converging (convex) lens. • Define the power of a convex lens and the dioptre. • Define linear magnification. • Construct ray diagrams to locate the image formed by a convex lens (Students should appreciate that all rays incident on the lens from the object will be focused, and that the image will be formed even if part of the lens is covered. • Distinguish between a real image and a virtual image. • Apply the convention “real is positive, virtual is negative” to the thin lens formula. • Solve problems for a single convex lens using the thin lens formula
Transcript
Lesson 4 • Define the terms principal axis, focal point, focal length and linear
magnification as applied to a converging (convex) lens.• Define the power of a convex lens and the dioptre.• Define linear magnification.• Construct ray diagrams to locate the image formed by a convex
lens (Students should appreciate that all rays incident on the lens from the object will be focused, and that the image will be formed even if part of the lens is covered.
• Distinguish between a real image and a virtual image.• Apply the convention “real is positive, virtual is negative” to the
thin lens formula.• Solve problems for a single convex lens using the thin lens formula
Lenses
Converging and diverging lenses
Principal axis
The straight line that goes through the centre of the lens at right angles to the lens surface
Focal point
Rays parallel to the principal axis, after being refracted by the lens, will all pass through a point on the principal axis called the focal point.
Focal length
The distance from the focal point to the centre of the lens (denoted by the symbol f)
Finding the focal length - a quick practical
Power of a lens
The power of a lens is defined as the inverse focal length. Power is measured in dioptres. 1 D = 1 m-1.
Ray diagrams
Standard ray 1
A ray parallel to the principal axis will go through the focal point.
Standard ray 2
A ray passing through the left focal point will emerge parallel to the principal axis
Standard ray 3
A ray passing through the centre of the lens will emerge undeflected
Ray diagrams
With the help of these 3 standard rays we can find the image of any object placed in front of a convex lens
Real and virtual images
• A real image is where the rays actually pass through the image and it can be projected and seen on a screen
Real and virtual images
• A virtual image is where no rays of light pass through, only their mathematical extensions. It cannot be displayed on a screen
Let’s try it! – Investigating images practical
Why?Let’s construct
some ray diagrams to
find out!
Thin lens formula
Thin lens formula
• f is positive for a converging lens• u is positive• v is positive for real images and negative for
virtual images• M > 0 means the image is upright• M < 0 means the image is inverted
Linear magnification
Linear magnification is defined as the ratio of the image height to the object height
Example problem
• A converging lens has a focal length of 15cm. An object is placed 60 cm from the lens. Determine the image.
Example problem• A converging lens has a focal length of 15cm. An
object is placed 60 cm from the lens. Determine the image.
• f = 0.15, u = 0.6• 1/f = 1/u + 1/v• 1/v = 1/f – 1/u = 1/0.15 – 1/0.6 = 1/0.2• v = 0.2 m• The image is real (positive v), m = -0.2/0.6 = -1/3which means the image is inverted and smaller
