Lenses Refraction of Light Lenses Word bank LENS
Lenses Refraction of Light
Lenses Word bank LENS A shaped transparent material (usually glass, quartz or plastic) used for redirecting beams of light There are two main types of lenses: converging lens and diverging lens
Lenses CHARACTERISTICS OF IMAGES FORMED BY LENSES A) IMAGE SIZE Images formed by lenses can be enlarged, reduced or of the same size as the object forming the image. B) ORIENTATION Images formed by lenses can be oriented either upright or inverted.
Lenses CHARACTERISTICS OF IMAGES FORMED BY LENSES C) NATURE Images formed by lenses can either be real (formed on the other side of the lens) or virtual (formed at the same side as the object).
Lenses Word bank CONVERGING LENS • Thick across the middle and thinner at its edges • Converges parallel rays of light to a single point called the FOCAL POINT • Also called a convex lens
Lenses Word bank DIVERGING LENS • Thinner across the middle and thicker at its edges • Diverges parallel rays of light creating an image in front of the lens • Also called a concave lens
Lenses PARTS OF THE LENS Vertical axis f Principal axis 2 F F f F 2 F F – Focal point 2 F – Imaginary point located twice the focal length f – Focal length: distance between the center of the lens and the focal point
Lenses PARTS OF THE LENS Vertical axis object f Principal axis NF f FF NF – Near focal point: Focal point closer to the object FF – Far focal point: Focal point farther from the object
Lenses RAY TRACING METHOD Similar to spherical mirrors, three distinct rays can be used to identify the image produced by a lens.
Lenses FORMATION OF IMAGE USING RAY-TRACING METHOD This process requires tracing of any two rays suggested below: 1) An incident ray from the tip of the object drawn parallel to the principal axis is refracted through the far focal point of a converging lens or the near focal point of a diverging lens
Lenses FORMATION OF IMAGE USING RAY-TRACING METHOD 2) An incident ray connecting the tip of the object and the near focal point of a converging lens or the far focal point of a diverging lens will be refracted parallel to the principal axis 3) An incident ray passing through the center of the lens travels straight on
Lenses FF object NF
Lenses object For diverging lenses you have to project the refracted rays back to locate the intersection.
SEATWORK An object is placed at different positions in front of a convex lens. Describe the image formed at each position by completing the table. Location of object Between the lens and the near focal point At the near focal point Between 2 F and the near focal point At 2 F Beyond 2 F Location of image Nature Orientation Size
Lenses LENS EQUATION Another way of locating the images formed by lenses is by the use of the lens equation: focal length, f: distance from the focal point to the lens object distance, do: distance from the object to the lens image distance, di: distance from the image to the lens magnification, m: the number of times the image is larger than the object height of object, ho height of image, hi
Lenses • • SIGN CONVENTIONS Focal length can be either positive or negative Focal length is positive for converging lenses and negative for diverging lenses Image distance can be either positive or negative Image distance is positive if the image is real and negative if the image is virtual Object distance can only be positive Magnification can be either positive or negative Magnification is positive if the image is upright and negative if the image is inverted If the magnitude of the magnification is: – Less than 1, image is reduced – Equal to 1, image is of the same size as the object – Greater than 1, image is enlarged
EXAMPLE A 12 cm high object is 17 cm from a converging lens whose focal length is 8. 0 cm. How far is the image from the lens? How tall is it? Given: ho= 12 cm do= 17 cm f = 8. 0 cm d i= ? hi = ?
SOLUTION di = (dof)/(do- f) = (17 cm x 8. 0 cm)/(17 cm - 8. 0 cm) = 15 cm (Image is located 15 cm from the lens) hi = (-di/do)(ho) = -(15 cm/17 cm)(12 cm) = -11 cm (Image is 11 cm tall and is inverted)
HOMEWORK 1) An object 5. 00 cm tall is placed in front of a converging lens whose focal length is 22. 0 cm. Describe the image formed in terms of its location, nature, size and orientation. 2) An object 5. 00 cm tall is 10. 0 cm from a concave lens. The image is one-fifth as large as the object. What is the focal length of the lens?
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