Colour dispersion with a prism (Item No.: P )

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1 Teacher's/Lecturer's Sheet Colour dispersion with a prism (Item No.: P ) Curricular Relevance Area of Expertise: Physik Education Level: Klasse 7-10 Topic: Optik Subtopic: Farbenlehre Experiment: Farbzerlegung mit einem Prisma Difficulty Preparation Time Execution Time Recommended Group Size Intermediate 10 Minutes 10 Minutes 2 Students Additional Requirements: Experiment Variations: White paper (DIN A4) Protractor Ruler (approx. 30 cm) Keywords: Task and equipment Information for teachers Additional information The aim of this experiment is to investigate the dispersion of light. In the course of this, the students' understanding of the refraction of light on crossing from air to glass or glass to air, respectively, is revised with reference to the dependence of refraction on the colour (wavelength) of light. In an additional part of the experiment, the question of whether spectral colours can be further split with a prism is investigated. In this way, the physico-historical aspect of the finding of colour dispersion is touched upon. For elucidation, the following two attempts at explanation should be given: J.W.v Goethe ( ) "Colours arise from the influence which has been added to the the glass body, so to say, by a pollution of the glass". Isaac Newton ( ) "Light which produces an impression of white in the eye is put together from many sorts of light which - separately, considered - arouse the sensation of different colours". In this way, it is possible to evaluate the experiment under consideration of the role of experiment in the development of understanding in the natural sciences and for leading to conclusions about the possibility of re-uniting spectral colours. Suggestions The co-ordinate system and the guide line for positioning the light box are necessary to obtain reproducible results and a nearly error-free experiment. The adjustment of the light beams by rotating the light box slightly may require help from the teacher. In this experiment, the use of a single-slit is intentionally avoided at the beginning in order that the colour-intense spectrum can make an impression on the students. However, the non-refracted component of the light will certainly interfere due to the large openingof the light box. The light box is therefore laterally covered. e.g. with half of the single-slit or with a piece of paper. If the supplementary accessories is available (Order Nr ) then an appropriate limitation of the light box opening can be achieved with the moveable screen included there. With the help of the single-slit, which is held in the light path of the coloured, fanned out light beams, further individual spectral colours can be faded out and investigated with reference to the possibility of further splitting of light. Remark on Result - Table 1

2 Teacher's/Lecturer's Sheet The students' drawings should be similar to that in the figure below. The markings are omitted for the sake of clarity. Remark The importance of reflection of the dispersed white light on the "back wall" of a raindrop can be discussed with the students: For the colour dispersion of sunlight, it is the refraction at the boundary between air and water which is, however, decisive. The diffraction which occurs should be neglected in the class discussion.

3 Colour dispersion with a prism (Item No.: P ) Task and equipment Task White light is made of many colours Investigate the colour splitting (dispersion) of white light following refraction at a prism.

4 Equipment Position No. Material Order No. Quantity 1 Block, rectangular triangle Block, trapezoidal , 4 Light box, halogen 12V/20 W Additional material PHYWE power supply DC: V, 2 A / AC: 6 V, 12 V, 5 A White paper (DIN A4) 1 Protractor 1 Ruler (approx. 30 cm) 1

5 Set-up and procedure Set-up Attention! Ensure that the position of the trapetoidal block is not changed on moving the light box. Set-up In the left third of a sheet of paper, draw right-angled, crossed lines and mark their intersection point as M. Make a mark 6 cm above M on the perpendicular line. Draw a guide line starting from M at an angle of 28 to the horizontal line. Fold the upper right corner of your sheet to form a screen. Fig. 1 Lay the trapezoidal block (rough side underneath) on the perpendicular line between M and the marking. Draw the outline of the prism. Position the light box without apertures, with the lens side at an angle to the block above the horizontal line. Fig. 2 Procedure Connect the light box to the power supply (12 V AC) and switch it on.

$Observe the refracted light beam leaving the prism as bands and correct the position of the light box by carefully turning it, if necessary.$ $The position is correct when the refracted light beams appear totally coloured on the screen and the violet part of the spectrum is also visible. Fig.$

6 Fig. 3 Now move the light box until the lower edge of the broad light beam is aligned with the guide line. Observe the refracted light beam leaving the prism as bands and correct the position of the light box by carefully turning it, if necessary. The position is correct when the refracted light beams appear totally coloured on the screen and the violet part of the spectrum is also visible. Fig. 4 Cover approximately half of the light box opening so that the light beam emerges only from the slanted surface of the prism. Note in the report which colours are visible on the screen. Fig. 5 Now carefully slide, from underneath, the tip of the right-angled prism into the coloured, fanned out refracted beam.

What do you see behind the tip of the second prism? Note your observations. Fig.

Observe and again note your observations. Fig. 7 Remove the right angled prism from the paper.

$Mark the incident light beam and the middle and edges of the refracted light beam and note in the report the$

7 What do you see behind the tip of the second prism? Note your observations. Fig. 6 Repeat this procedure, but now slide the tip of the right-angled prism from above in the light beams. Observe and again note your observations. Fig. 7 Remove the right angled prism from the paper. Insert the single-slit into the light box's lens side end. Shift the light box until the light beam fall approximately 1 cm parallel to the guideline on the prism. Mark the incident light beam and the middle and edges of the refracted light beam and note in the report the position of the colours on your sheet. Fig. 8 Switch off the power supply and remove the light box and the block from the paper.

8 With the help of the markings draw the path of the light rays before, in and behind the prism.

9 Report: Colour dispersion with a prism Result - Observations 1 (10 Punkte) Note the colours on the screen. Result - Table 1 (2 Punkte) Note your observations. Position of the tip of the right-angled prism In the red region In the entire light beam In the blue region In the entire light beam

10 Evaluation - Question 1 (10 Punkte) What happens to white light which passes through a prism? Evaluation - Question 2 (10 Punkte) Which colour is the most strongly refracted, which the least?

11 Evaluation - Question 3 (10 Punkte) Can the spectral colours be further split by a second prism? Evaluation - Question 4 (10 Punkte) Where can you observe similar colour phenomena in nature?

12 Evaluation - Supplementary problem 1 (10 Punkte) Supplement your drawing with a line having a perpendicular angle of incidence and try, on the basis of your observations of the narrow light beams and with the help of the law of refraction, to explain their course on striking a prism.

Complementary colours (Item No.: P )

Teacher's/Lecturer's Sheet Complementary colours (Item No.: P1066300) Curricular Relevance Area of Expertise: Physik Education Level: Klasse 7-10 Topic: Optik Subtopic: Farbenlehre Experiment: Komplementärfarben